I have an android phone, and do not have any drop down options to share within other apps. It goes straight to Facebook
Well, Lightroom mobile must have remembered Facebook as default share app, that's why its taking you directly to the Facebook.
You can reinstall the Lightroom Mobile App and it should show you the choices for sharing.
this is my problem too!
I would like to share pics on instagram, others on whatsapp. reinstalling the app sounds like a horrible solution