it breaks the ice and gives you a opening to start up a conversation, also, if a girl randomly messages you saying "do I know you" it's pretty much guaranteed they think you're cute. here's some excepts from messages where this was used.
HB
Hey do I know you??
Me
Not sure. ha, you may have been a random add.
HB
oh hahaha okay I was like ummm I feel bad but I don't remember meeting this guy lol
Me
no problem. yeah, I think if we'd met we'd remember each other. I don't usually forget a pretty face.
HB
Ohhhh smooth lol
Me
Just a little. you should give me your number though. Makes it easier to get to know each other.
Other HB- also ongoing
Me
hey HB. whats up?
HB
heyyyyy not being rude, but how do i know you? you don't look so familiar.
but im just watching tv.Me
I'm actually not sure. was hoping you could tell me. haha
HB
haha, oh. idk!
Me
well, you keep popping up on my profile and my home page so that's a bit odd. nice to meet you though. what's the show?
HB
real house wives new york, lol.
i don't get on fb much so that is odd!
Me
ah, I don't watch reality tv, so not familiar with that.
HB
well me and my mommy are watching it soo hahah
Me
hmm... you heard of Game of THrones?
HB
i don't think i know what that isssss lol
Me
the best TV show ever made. unless you have HBO you either have to pirate or get the sets though.
, Anyways I saw you the other day and I thought you looked interesting so I added you". And I n closed in around 15 messages. Her saying do I know you is an ioi. She wants to get to know you 
if she didn't she wouldn't have responded bro. Good luck.