HMU stands for hit me up, which is slang for telling somebody to call/text or contact you in one way or another.
"ay bro hit me up later when you get a chance i gotta talk to you."
"Hit me up tonite."
"I'm bored, someone hmu".
HMU is used a great deal on the internet specially on Facebook. In fact in 2010 this was the term found most in people's statuses, with 1600 a day having the term in them.