The only stable life-long identity system I know of are state-assigned identities (with the court system as a last line for complex cases).
Apart from that, I think per-service identities are the best that can be hoped for. If I choose to engage with Facebook, I have to trust them to some extent anyway, so trusting them with my identity on Facebook is probably good enough. If I want to establish that some Twitter identity is the same as some Instagram identity, I can do so by directly referencing them from one another. I don't think we can do much better than that without involving the state.
Apart from that, I think per-service identities are the best that can be hoped for. If I choose to engage with Facebook, I have to trust them to some extent anyway, so trusting them with my identity on Facebook is probably good enough. If I want to establish that some Twitter identity is the same as some Instagram identity, I can do so by directly referencing them from one another. I don't think we can do much better than that without involving the state.