The transformative scale blog series has aimed to spark conversation around the cutting-edge notion of achieving impact at a scale that actually solves social problems. Sixteen leaders, and many commenters, have shared their perspectives—what have we learned?

For me, two themes stood out: You can't achieve massive scale without the help of many others, and demand is as important as supply—rarely is it as simple as "build it, and they will come."

Don't go it alone.

Most contributors talked about partnerships and collaborations for scale, emphasizing the fact that going it alone is simply not an option when massive scale is your ultimate goal. For example, the Robert Wood Johnson Foundation sees corporate partnerships as essential to the success of its new Culture of Health initiative. "We increasingly are looking to businesses as partners to create scalable solutions—a natural fit, given that health has significant financial implications as well as social ones," said President and CEO Risa Lavizzo-Mourey.

Share Our Strength CEO Bill Shore concluded, "Large-scale transformative social change requires distributed ownership across many partners and stakeholders." Shore has built a national coalition of organizations to support the No Kid Hungry campaign to expand the government subsidized school breakfast program.

"Building a global movement to preserve biological diversity has challenged leading conservation organizations to reframe their thinking from protecting nature from people to conserving nature for people," said Steve McCormick, former president and CEO of the Nature Conservancy. This new way of thinking has led some NGOs to partner with corporations "to pursue the congruence of business and ecological sustainability," said McCormick.

SHIFT Academy Founder Mark Bonchek put partnerships and collaboration into a historical context. "The 21st century is an era of mass collaboration," he explained. "Scale is no longer about production but impact. It's not about how big you are and how efficiently you operate. It's about how well you catalyze action around a shared purpose." In short, massive scale requires a platform that inspires and supports others.

Multiple organizations have worked with others to scale their work—Year Up through community colleges, VisionSpring through BRAC, Health Leads by partnering with health systems, Teach For All through a global network of partners, Ashoka by promoting coalitions of changemakers, and Encore.org by teaming up with AARP.

Design with cost in mind.

The second theme centered on demand. Social entrepreneurs necessarily start out focused on the supply side. First things first. They have to build a model and prove that it works. But just because you create a program or service that succeeds in Peoria doesn't mean anyone in Poughkeepsie cares.

What does it take to create demand? Contributors focused on creating a winning value proposition at minimum cost. Unfortunately, social programs in the United States often aim to develop something that works, then fundraise to support it; but left out of this equation are upfront considerations of per-unit cost, the ability of others to pay, and return on program investment.

These factors are very much on Year Up CEO Gerald Chertavian's mind as he thinks about how to take his organization to the next level. After a decade of persistence, Year Up now mentors and trains 2,000 disconnected youth per year, yet 6.7 million young people remain out of school and out of work. Chertavian needs a new funding model that appeals to "natural" payers—employers, government, or young people. Private-sector employers clearly have the resources but need to see real financial return on their investment. So what's the right per-student cost, and how can Year Up show that its graduates are more effectively trained than other job seekers? Answers for Chertavian can show the way for others working in the youth employment field.

When organizations design for scale from the very beginning in the developing world, cost is one of the main constraints. By keeping program costs low, BRAC, the world's largest NGO, has managed to reach 135 million people. For example, in Bangladesh, BRAC set up schools in impoverished rural areas and trained local women who had not even completed high school to be effective teachers. At $36, the annual cost per student is affordable even for families at the bottom of the pyramid—and BRAC USA CEO Susan Davis shared that the schools are achieving better results than government schools.

Harvard Business School lecturer Michael Chu framed the social sector demand discussion in market terms: "In all of history, there has been only one way to create a new industry: develop an economic activity that earns above-average financial returns." High returns attract followers—other entrepreneurs and investors. "And without followers, depending on one single player makes it highly unlikely that the resulting impact will be sufficiently large to cause systemic change."

Markets, however, are not the answer for all social problems. As Charles MacCormack noted, donors of all sizes will continue to play a major role in funding nonprofits that deliver vitally needed goods and services. But he finds fault in the fragmentation of most funding, which makes developing a coherent, large-scale strategy all but impossible.

Regardless of strategy, prioritize execution and listening.

While collaboration and demand emerged as major themes, several leaders raised important issues that, handled badly, can derail any social enterprise.

Whether it's a program, a process, or a set of principles behind a push for transformative scale, results depend on strong execution. It's incredibly difficult to get this right and sustain it over a period of time. I found Curriculum Associates CEO Rob Waldron's advice particularly compelling: "When it comes to an organization's ability to succeed, no operating lever outperforms making the right hires." As CEO of a fast-growing organization, Waldron spends more than half of his time on recruiting.

One of the interesting puzzles that emerges from the series is the degree of focus needed for achieving transformative scale. On the one hand, organizations may need to follow multiple paths to achieve massive scale; others may need to focus on one. DoSomething.org leaders Nancy Lublin and Aria Finger observe that "getting small to get big," cutting back on programs, is at the heart of their organization's scaling strategy.

Finally, organizations can underestimate the power of listening carefully to their beneficiaries. Tostan Founder Molly Melching has spent more than two decades perfecting a technique she calls "responsive listening," a practice that guides the organization in working alongside, rather than for, its constituents in East Africa. Responsive listening, said Melching, "is actually the most essential element of helping a grassroots movement grow. For us, responsive listening has shaped not only what we do, but also how we fund it."

This series has advanced our collective thinking about transformative scale. More than ever, we view it as the social sector's new frontier. We hope you will follow the conversation on the Bridgespan.org resource page, where we will continue to explore issues and thinking around solving society's most intractable problems.

Jeff Bradach (@JeffBradach) is co-founder and managing partner of The Bridgespan Group. Before joining Bridgespan, he was on the faculty of the Harvard Business School.