How did he always know when I was on Facebook?

[.]

OK, so I used to be "friends" with this guy (it's been a very long time since I've last spoken to him). He actually lived across the country from me, and I swear, the second I logged into Facebook, he'd start messaging me. Like, I'm not even kidding. The very second I'd log in, he'd start messaging me. Like, the VERY second. It didn't matter if it was 1 am, he'd somehow start messaging me the very second I logged in.
I was wondering if perhaps he had some sort of "device" or maybe an alarm sounded off whenever I logged in?
Thanks

[?]

Well you can see when someone gets on because there will be a green dot by your name when you get on. That guy probably doesn't have a life and is on Facebook 24/7. Just ignore him. If you don't respond then he will give up eventually.

[Pearl L]

he might have something that tells him when youre on