Two users have created great comparisons of Google Plus, Twitter and Facebook, in table and infographic forms. I’ll show the infographic first, and below that, the table.
The infographic is by Stefano Epifani, who made this English version available from his (Italian-language) website here. Click to enlarge:
Hutch Carpenter (VP of Product, Spigit) posted this great cheat sheet as a table on Quora to highlight where Google+ is like either Facebook or Twitter.
As Jonathan points out in the comments, Twitter offers near-inline graphics and media, and should probably get at least a 1/2 for that 
