The problem? They don’t know about each other.

Google, today, with its new Social Graph API, is trying to hook all of those together.

Another problem? If you’re a developer and build a “foograph” your new application doesn’t have anyone on it. There’s nothing lamer than a new social network that has no one on it.

Google’s new effort lets you import friends from other social networks into your new application.

The details are on the new Google Social API site (the sample apps should be turned on shortly but weren’t working when published this blog post) with more background from Brad Fitzgerald on the Google Code blog.

The problem is that for apps to be able to sense all your connections you’ll need to add (rel=”me”) tags.

Mike Arrington at TechCrunch has a few more details on this announcement.

In the test that they ran for me it showed about half of my social networks. I expect that will increase pretty rapidly as more social networking systems support the “rel=”me”” system.

UPDATE: Plaxo just shipped the first app (they claim) using this new API.