I think that the only domain where social networks can be relevant is in the geoloc domain. Dating apps are quite an obvious example where it works very well, but dating is a very limited scope. So of course, the single problem of anything geoloc is safety and privacy, which is not the strong aspect of any social network right now. Tinder and others work hard to make their users safe, and it's difficult because users…
I think for dating/sex people are willing to leave their comfort zone and meet new people/take some risks. I not so sure about boardgames and walking... who wants to play a boardgame with a complete stranger ? but hey, why not? someone should try with the hope to become the new meta in social networking.
Parents with young kids may have other friends with similarly aged children, but many don’t. It can be isolating because so much of your non-work life/time becomes child focused. Kids that age are too young to have friends from school or activities. Some can strike up conversation at parks, but many find the inertia too much.
Good foundation to expand into clothes/toys reselling/gifting, babysitting co-op scheduling, nanny share, etc. which could all be monetized .
Probably a good niche for Facebook though I would rather see someone else do it.