Social media platforms contain within themselves the seeds of their own destruction, but that's pretty much been the case for the past 30 years, going to BBSs and Usenet.
There are a number of dynamics at play.
One is the Yogi Berra / Evaporative Cooling effect. "Nobody goes there any more, it's too crowded", or the more precise construction given at Bumblebee Labs. High-value members leave, the network is dominated by lower-value providers. As the Tilly-Odlyzko refutation of Metcalf's Law notes, not all contributors are equal. Network value doesn't rise with n^2, but n * log(n). I'd add on top of that a network cost function which is effectively constant per member, so we end up with V = n * log(n) - kn, where k is some constant cost. At the point where k > log(n), additional members lose* you value. This happens through spammers, griefers, trolls, scammers, idiots, and other forms of annoyance.
(A corollary: to grow your network, you've got to minimise that per-user cost -- it matters more than value ultimately.)
http://blog.bumblebeelabs.com/social-software-sundays-2-the-...
There's accumulated technical debt. Existing systems have an ever-growing component of klugey old software that's got to be maintained. Possibly even and Amdahl's Law (or Baumaul's Law) component where that comes to dominate constraints -- it's so baked in you cannot easily extricate it, and all the easily extricated stuff's been eliminated.
There's the falling costs of information technology. About an order of magnitude every ten years. Facebook cost about 1/10 what Google did for equivalent hardware functionality, Facebook's competition will cost about 1/10 what it did, and 1/100th what Google did. Much of that is eaten up through the Jevons Paradox -- making a thing cheaper creates an induced demand for more use. But there still seems to be a tendency for computing hardware to be ever-more democratised. The current challenges aren't in system capabilities, but in interfaces, and the compromises those present. It's the keyboard and screen size I need now, and the device I'm using now (a tablet and highly defective Logitech Bluetooth keyboard) is almost _all_ keyboard and screen. The latest generation of Apple laptops are essentially an iPhone slipped inside a keyboard/display case, with all the empty space filled with flat, form-filling batteries.
The larger challenges are relevance, the social graph, the interest graph, truth, and quality content. Filling a datastream with crap 24/7/365 is easy. Filling it with genuinely relevant information, and avoiding abuse, harder.