My honest take is that it's likely both. Increased social stakes is a big one, but it's incredibly difficult to build. It's hard to convince someone to use your app consistently, it's even more difficult to convince 2+ to keep each other accountable. My point is that it might be difficult to test in isolation, or even lead with this as the primary strategy. What you should achieve first is Sean Ellis test (something my friend from product taught me once) - answering single question “How would you feel if you could no longer use this product?” When this test holds (roughly >= 40% answer they'd be very disappointed), then it enables sharing with others, which enables social components you're describing.