In reply to Andrew Sun.

I've been working with Tomas and John Jacoby on this - we also have HHVM running with BuddyPress and it works fine for a very small amount of concurrent users. If we put any kind of load on it, HHVM crashes. It is very strange. We have some custom tuning of HHVM that was put in place after working with the Facebook engineers that makes WordPress core blazing fast, but it could be incompatible with BuddyPress - this is the kind of thing we're working to figure out.