In technical terms, the day and month that you entered this world, by exiting your mothers womb.
In Facebook terms, the day when people that you don't know decide to write a somewhat personal message on your wall, congratulating you on this occasion
Charlie: Happy Birthday Ashley! hope you have alot of fun tonight! maybe we can see each other later
Ashley: Charlie, who the hell are you?
Charlie: We met at Emily's party three years ago! remember? I introduced myself, you said go away I have a boyfriend, and then sprayed pepper spray on me? I sent the whole night crying in the bathroom!
Ashley: oh yeah sorry about that. By the way I dont have a boyfriend anymore. You wanna get together later?