How to Keep Facebook Chat active

[rocknstomper]

How to Keep Facebook Chat active, when on standby/lock screen.
I just got this phone installed Facebook app and Facebook messenger. I sign in using phone on both(says *mobile) to others then signed in only one(each one). I push the standby/lock screen button and its goes offline (friends see offline). I was hoping there is a way I can keep it online while on standby/Lock screen.

Thanks for any help
I've searched all over and can not find a srt8 answer

Joseph