Since Google just brought out its new reading age filtering tool, I've used it to compare the main English-language social networks.
There's almost no pages classed as "basic" on LinkedIn and almost none classed as anything other on MySpace. Inbetween, there's some odd results though - Livejournal, the home of some pretty venerable and serious web diaries, is almost as basic in its overall language as MySpace and Bebo. (Maybe it's because all the serious journals are locked as friends-only, but that's nothing more than an educated guess from a long-time Livejournal user.) Twitter has the second most intermediate language and the second least basic, as befits an increasingly professional news and comms tool. Finally, Facebook has more advanced language pages than any newspaper (or indeed any other site I've been able to find - Google classes a massive 22% of its pages as "advanced" - here are the advanced results for you to see for yourself.
The reading level results are fairly consistent with danah boyd's research on social class and social networks - Facebook for the middle classes, MySpace for the proles, LinkedIn for sounding professional. But that 22% advanced rating for Facebook has me frankly baffled - suggestions for what's going on there very much welcome.
Notes on methodology: the main www URL typed into Google Advance Search with the "annotate results with reading levels" option turned on. Different results seem to be generated by missing out the www, in some cases. Results are ordered by % of pages classed "intermediate". HT to Searchengineland for the original pointer that the tool exists.