So, i've been playing osm for over 1 year and i've connected to facebook via phone. i've also been on osm on my computer via facebook, but still no achievement...
CONNECTED TO FACEBOOK ACHIEVEMENT NOT GIVEN!
I'm not sure how you play on Facebook, the only thing I know is that this account is not associated to any facebook account, reason why you can't claim the achievement.
In order to associate it, just open OSM on Facebook and when asked if you want to create an account select the option I already have an account and log in with this account data. With this I'm sure that you'll be able to claim the achievement!