I've figured it out all those pics i see at the top are people i have had private conversations with some of them are no longer here

That would make sense since it's not based on who is online. I've seen people pop up there whose profiles have been 'away/taking a break' for over a year or who simply haven't been online in that same period of time or longer

I think the first 3 or 4 people on the top header makes sence but then after that not so much.

It's unbelievable that people who have not been here in a couple of years are showing up there