When it comes to Social Software in the Enterprise the full suite solutions are on their way. See the last Gartner Magic Quadrant for Social Software for details and vendors.
As the pure products vanish and build up on functionality, it might be worth a look to some general categorizations of social software. So we won’t have to talk about “kind of a blog with wiki functionality” or something similar.
Gartner uses the dimensions “ability to execute” and “completeness of vision” in the Magic Quadrant which are great for vendors or products, but not for functionality in general.

A common approach in germany builds upon a classification system for CSCW-Systems. (Teufel, 1995)
The first adoption for Social Software by Schmidt focused on the three funtions Informationmanagement, Identitymanagement and Connectionmanagement.
Social Software Triangle by Schmidt
In an improved version by Koch and Richter (Cooperation Systems Center Munich (german), Bundeswehr University Munich) changed the connection part to communication and added the loose connections to the identitymanagement. You might think of all your quiet facebook friends here.
Social Software Triangle by Koch/Richter

Niall Cook has a totally different Matrix, the 4C’s as in his book Enterprise 2.0 book.
Social Software 4C
I merged two diagrams to get this one, so some software examples aren’t in here. I’m not confident with this classification as there are some well known apps split up across the matrix, i.e. Tagging and Social Bookmarking. In the original book you’ll find more examples.
Cook mentions cooperation and collaboration, two points which misses out on both triangles.

Another idea is a draft by Joachim Niemeier in a german slideshare presentation.
Social Software Quadrant
Personally I like the quadrant best, but I would add some modifications to it. As long as my ideas on this are not fully set, I prefer even more inspiration.

So did I miss out some well known ideas?

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