Kudos for releasing the app! IMHO, Unless you're married, don't put your relationship status on FB. Set it as hidden, or say nothing. When things change, and in life they do... the people from whom you don't want to hear are the first to notice and / or comment on it.
Depending on your SO - you might not get that chance. It's not a problem I'm likely to get but I can just imagine the question 'why haven't you updated me as your girlfriend on facebook?'
If you just leave it off, you can at least make a case out of principle for it.