I see your point, and agree, well said, but not all social networks/web apps should aspire to be the next Facebook, some of them should try to do something different, not necessarily for the masses.
I think the huge challenge in these sites is that they start off simple and targeted (like Facebook), then grow, and find they don't have a sane way to deal with that growth.
Of course, trying to solve it before you need to is just a pointless waste of time, because odds are very against you ever needing to solve it.