That'll require a lot of cooperation among fierce competitors.
inter-website match!

Against these guys maybe?

One difficulty is that the other website will have to join chess.com in order to play - but then THEY'LL be members of chess.com - so we would have to look for ANOTHER website to play against.
And so on.

it appears that there was some matches in the past:
http://www.ficgs.com/Match-FICGS-vs-GameKnot-q994.html
Hello! I just had an idea that maybe the awesome members of chess.com (or maybe just the masters) could play against other websites in an inter-site match! wouldn't that be exciting?