I already mentioned this in another comment, but it's also relevant here: I don't understand why it's assumed that contact and identity isolation should lie at

I already mentioned this in another comment, but it's also relevant here: I don't understand why it's assumed that contact and identity isolation should lie at the service provider boundary. People are able to manage multiple email addresses and phone numbers for work/personal/whatever without too many problems, and despite being against Facebook's terms of service many individuals already create more than one Facebook account to keep stuff separate ("I don't want my boss to see photos from that party last weekend"). In a standardized IM interop utopia I'd imagine that juggling multiple identities would be even more common.