Idk man i dont usually use facebook for sarging, because i find it way too impersonal...
But i mean when a girl does something like this she usually wants to get back to a higher standing with you..
What i would do is comment back like... "Alright Mrs. Random, i havent seen you in years how have you been

" (in a jocking way) Or something along those lines just to try and find some connection between you too.
What i found is that when a girl hasn't talked to you in a while is because she found a BF but when they ramdomly messege you like that its because they either broke up or they're giving it a break (make sure you know all the factors)
At least that's what i learn from past experiences.
Hope my opinion helps brotha