Sometimes/Sometimes (and "other," of course.

) I think I've been invited more than I've invited, because I keep forgetting the feature is there. (oops.) I don't believe I have ever turned down an invite.
I'd like to see a "Post by Friends" view that would show me all the threads my friends have been active in, so I could keep up with you, but then again, I can't even keep up with the threads I'm already participating in.