Where do U.S. teachers hang out online?

Here in the UK we have a great online forum called TES Staffroom, which claims a membership of over 200,000 (out of a teaching population of 500,000). I have tried in vain to find its U.S. equivalent. Is there one? Are there several? I'm excluding C2.0 itself because it sits on the leading edge. TES Staffroom is very mainstream, and that's what I am trying to find across the pond. All pointers will be gratefully followed!