Facebook last active time?

[Nick]

So my girlfriend says that on Facebook me & this other girl are always online & last active at virtually identical times & she's got suspicious. She says we're always online for about 30 mins, then both go offline within 2 minutes of each other. Then we both come back online at identical times & go offline 40 minutes later. It also says we go to bed within 10 minutes of each other. It also says I am online when I am not even on sometimes. The thing is, I talk to this girl a little bit, like once every couple days but that's about it.

The thing is it's got my girlfriend worried. Do you know why these weird times happen? Is Facebook's last active feature really unreliable? Does it just guess? I just want to give my girlfriend a proper logical reason why it happens, because just my word won't prove anything