There are a few federated platforms that are doing pretty well in terms of development, and they tend to have distinct sets of users. Some are bigger than others, but the platforms in active use are:

* Diaspora - https://diasporafoundation.org * Friendica - http://friendica.com * Hubzilla - http://hubzilla.org/ * Libertree - http://libertreeproject.org/ * GNU Social - http://www.gnu.org/software/social/ * Pump.io - http://pump.io/

What's interesting is that the first three platforms can all federate with one another, and there is an established network that acts as a community - Libertree is discussing implementing compatibility with them.

There is also a wider effort by a W3C group to develop a protocol that could work across all of these platforms, provided that they all follow the same privacy standards.