I don't have a Facebook account, but my understanding is that it supports app integrations, and that connected apps have access to the account information you choose to share with them. Why wouldn't this work for Urbit? And why, in theory, wouldn't Urbit be able to support a distributed Facebook in the same way that GNU social supports a distributed Twitter?
The concerning part would seem to me to be how you cope with a situation where you have some people hosting their stuff on their urbits, and others still on Facebook; I don't see a solution here that does not require technical cooperation from Facebook, which as you rightly note will not likely be forthcoming.
As I know to my cost, people stick to Facebook because Facebook owns their social lives, so people aren't going to risk trying to emancipate themselves from Facebook unless they can do so without maybe losing all their friends. No one at Tlön has articulated anything like a clear solution to this problem, and I think it would be a very good idea if they did, because it will kill them if they don't watch out.
Urbit is provably effective at generating techie amazement, and I can well believe it created enough for well-off people to spend money on address space for the amusement value (at least one has explicitly said that's why he bought some). Urbit is a legitimate mad art project, an untrammelled 200-proof expression of its creator's personality; that is in fact interesting.
This is not, however, the same as usefulness.