In addition to the above, David Millar send an excel file that he started using in the beginning of last year. The idea came up (during yesterday’s online session) that it might be nice to start from this and other documents to build a matrix on. Here is David's excel file: EEwebsites_davidMillar
Recently I got hold of this wiki-matrix which compares wiki's really nicely.
I also found a matrix which focuses on CMS's.
Who knows of other social media apps matrixes that might help?