Did you ever post a link to photos on Facebook? Does your one Bloop friend on Facebook have other Bloopers on HER Facebook? (If so, they could find you via comments she's posted on your photos). Even if Facebook profiles are private, your friends list is public unless you change your Facebook settings. And your photos are usually visible to friends of friends if they comment on them, or you tag them... unless you change those settings too.
So several ways in which people could have figured out which one your boyfriend is. Still, would have had to be someone who knows either you, or your friend.