Find a conversation that they had with their girlfriend and or boyfriend (the more obscure the source, the better, make them wonder how you heard it), and, with your partner, strategically recreate it on the target's Facebook page.
Dress like them and impersonate them
If they have really cheesy pictures of themselves that they posted on Facebook, take the same picture. With you, though, of course. Don't forget to tag the target!
If your target is totally in love with your best friend, make an awkward picture of the two of them and a cat, and stick it on a heart shaped ornament for the Target's Christmas present (I regret that I didn't take a picture of the ornament, all I have left is this picture, a gem nonetheless.)
Way back when I liked this one kid (who's name shall not be mentioned), my brother wrote me a song, which successfully made me feel mocked.
So you see, mockery is a great pastime. Just make sure that your target is not sensitive, weak, emotional, or girly in any way. Because that is just asking for trouble.