Persona is not a social network, though, just an identity system. But there has been talk of the Persona team potentially cooperating with the Tent (https://tent.io) team.
(edited to remove slangy word "rumblings")
Rumblings?
Is it a part of firefoxOS? It could be a great way to push browserID adoption.
Yep! As I understand it, FxOS will ship with a native implementation the Persona APIs. However, I don't believe the devices require any sort of attached identity until you try to make your first purchase in the Marketplace, so it's not like the unboxing experience will be used to bootstrap Persona.
It is part of the Marketplace (bundled app store) in Firefox OS. I don't believe Persona is the main identity system in Firefox OS at the moment, but it's very possible it could be in a future version.