I've only ever properly concluded one RP - also happening to be the only 1x1 I've ever had successfully. Man, that was a fun one. What sucks is that I can't find the site we wrote it on! (I used to make ProBoards sites for all my RPs, 'cause it was convenient and fun)

Me and a close group on another site ran one story for a long-ass time, but it was more of a "could have ended but we didn't want it to" things. It certainly didn't die early, so I consider it more or less complete. 

Other than that, nada. Although I have been a part of a lot of reboots. Even hen they don't live long, it's still fun to re-live those old stories.