In the actual photo? Is there any meta data? Is there any EXIF data? What was the photo filename, and what does searching for that (with increments up and down) return?
To try and guess what happened:-
1) One of them followed you and got information. (You say this didn't happen.)
2) They had information about the event and worked from there.
3) There were other people in the photo, and they got face-matched, and their friends lists were not hidden, and they got it from that.
4) They had snippets of other information and just trawled through very many searches to get the name.
I'm a bit confused how they didn't already have her name. You told them you met someone, but didn't give any kind of name?
To try and guess what happened:-
1) One of them followed you and got information. (You say this didn't happen.)
2) They had information about the event and worked from there.
3) There were other people in the photo, and they got face-matched, and their friends lists were not hidden, and they got it from that.
4) They had snippets of other information and just trawled through very many searches to get the name.
I'm a bit confused how they didn't already have her name. You told them you met someone, but didn't give any kind of name?