Some of the replies in this post and many of the those in other posts show why online poker is profitable.

You don't have to be great, or even a particularly good player to make money (just better than enough of those players who know obsolutley nothing, to out-perform the rake). I started 3 months ago, keep records and don't play full-time. I have put zero cents (zilch, nothing) into poker accounts; started out from free-rolls and no-deposit cash bonuses. I have taken out over $300 (not a fortune, but from 0.5 / 1 no-limit games it's not a lucky streak either).