Personally I think its nice to see this kind of creation tool decoupled from a social network. I am hoping it can take Vine like videos, which I can keep and redistribute in a method of my own choosing.
I'm interested in why you consider Messages a social network? To me, Messages is an iteration on MMS, analogous to the transition from SMS to MMS.
Messages can only be used by Apple customers, but this is not indicative of a social network and would detract from the value of a legitimate social network.
We would have to agree on a definition of social network, but I'd argue that being inclusive (not requiring a specific hardware) is not a prerequisite.
iMessage is not fully interoperable with MMS or SMS. Many messages are sent in a mangled form to non-iPhone users. If you deactivate an iMessage account, Apple will just sent group messages intended for that account into the ether, rather than sending them via SMS. There are lots of quirks that make it not fully interoperable.
It's exactly what we criticized Microsoft for in "embrace and extend" back in the 90s.
I imagine this will come with a Messages App though the UX around those integrations is terrible the standalone app will be enough.