From a technical standpoint, this is largely the same problem as the portability of other user data, with the added complication that the data must be signed by

From a technical standpoint, this is largely the same problem as the portability of other user data, with the added complication that the data must be signed by an identifiable originating party other than the user that the receiving party can correspond -- and establish a trust relationship with -- to verify the lack of tampering. Online services have been around for a long time and yet very few implement data portability. Often, it's part of their moat against competitors and ensures lock-in. Few incentives exist for incumbents to offer data export. From a social standpoint, this raises some serious issues. Distributed reputation systems exist informally, whose lack of universal applicability give them some protection against trivial malice. In the financial world, various credit scoring systems use trusted third parties to aggregate information from entities one has transacted with, and offer a service for other parties to obtain this information on the user's request. In some places government-issued strong authentication is used to establish identity, but in the US, weak identification and publicly available information is used as a challenge to the user to establish their identity. Both of them come with a host of issues that ought to be cause for concern. If no link to some "higher form" of identity is implied, then the problem becomes almost trivial: each user of 'Service A' can become a cross-referenced user on 'Service B' by proving control over their account in 'Service A'. But this too is vulnerable to credential theft, and new guises can be created at any time by just creating additional accounts in 'Service A'.