It's pretty late in the game, but somebody sent me an Orkut invitation (thanks Josh!) -- an explicit one, that is, I've had several people do the "if anybody wants" thing and just not taken them up on it. So, now I'm there (under the name you'd expect), in addition to tribe.net. Whee. I did a few friend-of-friend type crawls and found a fair number of the old school blog tribe representing. Yee haw.
The forum stuff looks less useful than on tribe.net (no tracking of what I've already read? Please!), but I like the friend network displays on Orkut better. I won't be surprised if six months from now, I'm not looking at either of them regularly. So it goes.