Brian Hendrickson has a point:
When small social networks like Twitter and Google Plus start to interoperate with open source networks and blogs, they could eventually form a large enough base of users to “flip the iceberg” and have more usage than the dominant, non-interoperable player: Facebook.
I don’t know if it will be any of these services, but I’m sure this will happen sometime in the near future. The W3C is actively pushing new standards like Webmention and Micropub to boost a more interoperable, „social network-like“ open web. My guess is that Google will be on there forefront of this movement, because they’re intrinsically interested in indexable and searchable content (and meta data), while Facebook keeps building walled gardens.
When this shift happens, Twitter will be an attractive takeover target again. And the day users are able to tweet to people outside of Twitter will be the first day of the end of the Facebook as we know it.