FB and Twitter have a weird stalemate they're both losing, and LinkedIn and Tumblr, respectively, benefit. Instagram is a good example of the FB/Twitter problem, it's the service FB wish they had built and Twitter wishes they had had sooner. A billion bucks for Instagram wasn't an example of a smart investment for FB's future, but a more defensive, even desperate move to buy a threat that was the future. While the walled cities compete for and buy userbase to feed their generally underperforming ads to, Google's focused on getting more ads in more places that dovetail with their core, particularly mobile, via their operating system, apps, and app store, and maintaining higher prices than the general social networks in spite of the huge increase in inventory by outperforming their competition, particularly by making remarketing easy and ubiquitous.
The probable future for FB is likely the fate of Myspace, Twitter will probably end up a niche microblog, LinkedIn will become increasingly relevant and valuable, and the not-so-dark horse who's trying to eat Google's lunch where they're not entrenched, and is succeeding in valuable ways, is Amazon. Ebay is where the lost potential is, they've proven themselves the opposite of agile and are best at alienating their userbase, if their culture wasn't so conservative they could put some deep hurt on all these other guys because for the foreseeable future they are the de facto online bank.