What was this?

I was on Twitter, and had a bunch of other tabs open, one being Facebook. Suddenly I see a message thing pop up on the bottom, right side of my screen. It's a Facebook notification thing, it showed my profile picture, it went away too fast for me to read it entirely but said something like I saw the number 2 and also 3, like some types of notifications. So I went to the open tab with my Facebook but it's not showing any notifications . I also didn't get any emails regarding anything. What could that have been a notification for? See I barely use Facebook, so I'm not used to getting many notifications for things, and I haven't gotten anything in months. So any ideas what it would have been for..... maybe something someone did but erased or something, or could there be anything else? What happens if someone tries to chat with you on there? Can they take it back if they send something, before you see it?