There is very little comparability. Last fall, chess.com decided that its standard chess ratings, which presumably are most like the over the board ratings, were so out of line with actual over the board ratings, that it inflated most ratings by several hundred points.
Here is one thread: http://www.chess.com/forum/view/livechess/standard-ratings-boost
Other people have tried to make comparisons between chess.com blitz ratings and USCF over the board ratings :
http://www.chess.com/forum/view/livechess/ratings-comparison-chesscom-blitz-v-uscf-otb
Here is another:
http://www.chess.com/forum/view/general/fide-ratings-vs-chesscom-ratings-explored
What are the ratings on chess.com compared to other sites and real life? Are they the same? If for example, you are a 1400 on chess.com, will you be around that everywhere else?