I see many people coming to my facebook game with HTTP referer that includes
fb_source=games_ego
Previously all options were at
http://fbdevwiki.com/index.php?title=Ref&cb=7347
but "games_ego" is something new.
What could it mean?
It's new section "Discover New Games" in the right block which is shown only in games.
Official Facebook documentation:
https://developers.facebook.com/docs/games/services/measurement#referraltracking