Now that Andreessen invested a huge amount in them, I'm counting the days until it's being acquired by Facebook. Seems to be his modus operandi. An easy 20x ROI with this strategy.
That's the whole point. They know that social networks are not stable in the long run. They are now a new type of tech company, one with a huge pocket (in stock), and willing to make big bets with it.
Facebook is a tech rollup, and in my opinion, if they keep absorbing the most promising startups (and allow them to run independently), they could be a trillion dollar company with FB the social network making up less than 10% of that market value.