They come from numerous sources: from players logging in through their console (Xbox, Wii, etc) to play the game, using apps on their phone to look at their teams and make trades, and logging in through the Web to check out the options market.
Unlike some Web-based phenoms, the location check-in service Foursquare has an obvious potential business model: local ads based on your geographic location, perhaps coupled with inferred interests from your previous check-ins.