Hi, just wanted to join discussing…
Facebook Groups offers now topic categorisations just like categories in discourse. To convince existing members of a Facebook group to go and use discourse is even more difficult. As one can say, hey why we need this, while we have all that already in fb groups 
Someone also said, that extra value, like content and conversation will differentiate, but initial conversation will not develop while they have already made that value in fb groups…
What would be on par with fb is as someone said one app, like Tapatalk but newer and better, with a functionality to subscribe to multiple discourse forums, manage your user accounts as aliases for main account (like @codinghorror mentioned for @David example) … and one BiG thing Push Notifications within app for all devices (iOS, Android)… Some brainstorming ideas of mine 
Facebook is dominant because 90% of people are using it by smartphones with otifications etc… we have to take some portion of this percentage by offering a mobile, responsive solution…