I'd like to make a bold wager. None of the big social web communities that are popular today will be so just ten years from now. My hypothesis is that massive improvements to network speed will reduce the pressure to colocate data. The traction we see to such tools is in communication, availability and connection to friends and those we seek to befriend. This functionality can be fascillatated better by moving away from the client/server model tied to RESTful design (how HTTP or the web is setup), and embracing peer to peer communication technologies. Federated network communications can take place without intermediary databases. There will always be a role for dedicated servers, but their dominance of attention in the future will wane as (mobile) Net participation skyrockets.
Fully distributed data undermines the central server ideology. The collection of all peers willing to share, defines a new federated database which is much richer than a central one. Large collections of peers can be dedicated to updating the state of the index (knowledge of what's in the database), mimicking modern data centers. Neighborhoods of knowledge will emerge as real time state is exchanged between members. In the peer to peer model, connectivity improves with each connected node (new paths). One of the major drawbacks of client/server models is limited write access to a centralized datastore.
But the social web has shown only an increase in user participation. When network data is freely available to all peers, central servers which tax transactions (without providing additional perceived value) will be bypassed. Why go through Facebook, or Twitter when I can communicate directly with other folks in an analogous way? Said another way, a company won't own the opportunity of connecting to other people, or "subscribing" to their stream. A remote, monsterous database doesn't own your friendship data (you can lease this info temporarily), nor does it defacto provide improved social efficacy between yourself and others. As peer to peer technology catches up, we will communicate and share information efficiently with all those who are interested without ever needing to route through a big corporate database.
The very nature of the web will uncover these paths without paying a premium to a connector. Those social tech companies that survive, will drastically alter their business models to focus on adding real value to users of their services. The network pull we feel today (peer pressure) will be disrupted by open and tax free data portability, commoditizing the social web.