Gf's Facebook always says online should I worry?

[Ryan]

I wounder if my gf is lying to me, almost every night her Facebook say she's online to chat ALL night I work at 4am.. She tells me she's going to bed around 10-11 and it will show her online till 12am wen I go to sleep then wen I get up at 4 for work it still shows her online and at 6 she's still on.. I asked her why she's up all night on Facebook and she denies it but that's the only time it does that during the day it's off and on as she is.. So she tryed to prove that she's not up all night and wen she spent the night she used Facebook on her phone and then put it away it showed her on line for about 2 hours and went of.. Is she lying to me and has this happened to anyone else? Thanks
She is on her cell phone but doesn't use the app she just goes to the browser on her phone and uses it