There is a relatively new social network site whose address is ning.com. It is designed for people to create their own site around their particular interest, and (so far) it has three plein air groups. One is by invitation only (Lower Hudson Valley Plein Air Painters) and the other two are open. The larger of these is called Plein Air Artists and has 637 members, with a page for photos (5,442 so far), videos (40), events (69), blogs (104), groups (24), a forum (42) as well as your own page and a chatroom.
The other group is Austin Artists which has 39 members including painters, sculptors, other fine artists in & around Austin, Texas.