Rather than keeping it and posting on Facebook to try and find the owners?
Seen yet another 'found this phone on the bus, if you can tell us what the home screen photo is of, or you can unlock it, you can have it back. If not, we will hand it in to the police after a week.'
Grrr, you should be handing it in to the police or the driver anyway! Fair enough to then post on FB to say where you found it, and where it's handed in, but they shouldn't keep the item themselves.
AIBU to think people that do this, do it to try and get the 'glory ' from the person who lost it in the first place?