I have LR on my Mac, and LR Mob on my iPad 4 and iPhone 6S.
I created a Collection which synced successfully between iPad & iPhone. I tried to export the Collection from iPhone to social media using the 'Open In' option but while that is there to select, it doesn't bring up a list of apps to export to... e.g. Facebook, Instagram etc.
I can see the social media apps list if I try to export a single photo, just not a Collection, which seems odd given the option is there.