Inspired by the Sentinel's Blogpost about whether the internet based communities can survive / be resurrected in the era of Facebook -
Clichéd though this will sound - I put it down to the "markets versus community" tension. Community and the relationships it fosters is a basic human need. In as much as community has been part of hominid evolution for hundreds of thousands if not millions of years (since the time of the baboon troupe), community is probably a defining element in the human environment of evolutionary adaptation (EEA). Human social interactions with and in its community are thus foundational to human nature - defining its distinction from monkey nature.
Industrialisation brought about migration to cities with nuclear families (disrupting kinship groups) and dense populations (too large to aggregate into a new community) - stable "village" community groups of the pre-industrial era were disrupted. No alternative to the stable - durable community of old emerged. Congregational religions - through church / parish based aggregations may have provided an alternative in some measure - but they too were no where as durable.
In the last 15-20 years - the internet provided the chance to aggregate interest based communities by wiping out geographical distance. It is debatable to what extent this could simulate the close-contact of pre-industrial communities. But internet communities have the advantage of avoiding the unpleasant side-effects of cheek-by-jowl community living.
Social networks tap into this very human need for community. But driven by the market dynamic of the network effect - where the larger the network the greater the benefit to those deriving revenues from the network - social networks soon grew even larger than post-industrial cities. Too large to aggregate as a community.
Nevertheless, social networks are by definition malleable, and shold allow for enough internal mobility for interest based communities to aggregate within the gigantic superstrtucture. So where as in an industrial city - say Shanghai - it would be difficult for a community of say dim-sum lovers to get together and decide to live together in a single city block; such aggregations are much easier within social networks. This is where I think Google+'s idea of "Circles" has significant potential.
The biggest downside of internet based communities can be their very freedom - where village-communities had natural barriers to exit, forcing people to adjust, accomodate and resolve differences - in online communities people can just up and away at the click of a button. Valuable social skills that develop through growing and living in communities may never be learnt!