As for the poker it depends what stakes you want to play, anything on the ongame network and Ipoker network have a decent (ish) amount of 1/2 - 10/20 running during peak hours (around 2pm - 4am) but ongame definitely tips Ipoker for regular volume.
Not played on any other software in the past 2 years so couldn't comment on any others.
Depends on what stakes but I'd keep a small roll on most networks and try to identify some regular fish. Basically the biggest ones are ongame and ipoker as already said, and some action on entraction aswell
