An explosive expression of love, so explosive, in fact, that the author, eager to convey the sentiment, types so fast that they neglect to remove their finger from the Shift key prior to striking the 3 key, thereby resulting in the common typo <# instead of the intended heart.
Roadkill heart. Meant to imply the torment and bitter anguish of extending love to a friend, only to have that love nastily snatched away, laid out on scorching concrete, run over by a Hummer (several times) until it becomes nothing but pulp-of-heart. With nice little tire marks for dramatic effect.
In rare instances, accidentally used as a typo of <3.