Prolly has become a popular slang for "probably" on chat/IM systems like Hotline, ICQ, IRC, and AIM.This is in part because of the way "probably" is pronounced (although in the states it is said more like "probly", and also because it is shorter than "probably".
Anything chat users can do to decrease the number of keys they have to press is apparently considered a Good Thing, evidenced by the decay of capital letters and the rise of abbreviations like LOL, ROFL, IMO, WTF, and countless others. An even more striking example is the shortening of "ok" to just "k", a one character or 50% increase in efficiency depending on how you look at it.