Boom; not the captions you see in the old Batman TV series, nor the song by P.O.D. Boom. But making a statement in something that you have done being extrodinary in saying the word "Boom!" after something has been done.
"I just won a free Big Mac, Boom!"
Or winning a hand at BlackJack "Boom!"
Nothing to do with terrorists and explosions going off. Just expressing yourself in a way other than an emoticon.
This was first seen on FaceBook by entrepreneur 'Mark Hastings', captioning every picture post with the word "Boom!". As we furthermore saw the trend move on to LMFAO in a few of their songs.