But those (or similar) communities existed long before Facebook did - they serve a vertical of common interests, rather than a horizontal of people you know. Some of them might be replaced by Facebook groups, but plenty of them will and can continue to exist outside of Facebook.
For any kind of specialized discussion on computer stuff and startups, I'll take a bunch of perfect strangers on HN over friends on Facebook any day. On the other hand, posting pics of my new apartment or from my holiday would be (let's say) underappreciated here. On the other hand, people I share friendship or family ties with cares about this stuff, even if we have no interests relevant to the matter in common.
For any kind of specialized discussion on computer stuff and startups, I'll take a bunch of perfect strangers on HN over friends on Facebook any day. On the other hand, posting pics of my new apartment or from my holiday would be (let's say) underappreciated here. On the other hand, people I share friendship or family ties with cares about this stuff, even if we have no interests relevant to the matter in common.