"HMU" actually stands for "hit me up." This means that whoever posted this abbreviation in their status on Facebook would like their friends to get in touch with them so they can all hang out. Friends can "hit up" their friends through messaging, texting or calling.
In 2010, estimates showed that the abbreviation "HMU" was used 80,000 times a day on Facebook. It was most popular during the summer months when students had more free time and there were more things to do. After the summer was over and the school year began again, the phrase decreased in use; during this period, it was found to be more popular during the weekends.