I was browsing some user profiles on the Politics.SE Private Beta (note: requires private beta access) and clicking on "network profile". Private betas are not shown on the network profile, which is probably intentional. However, for some users, the link from the private beta user page to the network profile leads to an error 404. I suspect those are accounts that exist exclusively on the private beta, and are not linked to any other accounts.
For example, see this user page (note: requires private beta access). From this user page, the network profile link is broken (error 404). I haven't been able to quickly find another example to test my "lack of linked accounts" hypothesis, because all other users I tested have linked accounts.