None, but evidently the point I was trying to make was a bit too elusive — it is the userbase and concentration of information it forms that is itself the use case or desirable feature. FB's walled garden IS the tool.

I don't myself agree with it - I'm in your camp, thinking there're far better existant tools out there. I just know from my own experience I'm 'missing out' on stuff shared or put in there by people locked into a domain I do not myself have access to.