And people pay $10/mo for Netflix, why wouldn't they pay a few bucks a month for an actually good social network? Heck, make it free for a month or six, no credit card needed, and when people see how good it is they won't leave. I pay for Disney+ just to let my kids watch Disney movies BECAUSE THEY'RE GOOD.
People won't subscribe to a non-free Facebook competitor because Facebook is free and none of their friends are on the non-free competitor... And most of their friends never will be.
Most likely the product will be free for a while and switch to paid. It kind of sucks but for a really good product it could work.
This is almost Reddit's model (with Reddit Gold, and now the various other comment/post awards added recently), except Reddit also sells ads; it'd be interesting to see if the awards alone are enough to keep the lights on absent a need to appease investors.
My response to that tired theme: you don't need a mega $B valuation to pay for a simple social network for sharing baby pics with friends and family. A non-profit or a benefit corp structure would be just fine.