Mailpile (https://github.com/mailpile/Mailpile) has 458 closed (3 open) pull requests, of which 334 were merged.
I think that's not too bad!
Although I do wonder what the numbers are like if I exclude PRs from the core team. Not sure how best to measure that.
(metrics prompted by thoughts prompted by http://sealedabstract.com/rants/nanomsg-postmortem-and-other-stories/, ht @Trev )