Techradar: The Sony numbers refer to PSN registrations which represent lifetime-to-date PSN registrations on both PS3s, PSPs and through PCs (although not by the same person across more than one platform)," according to Screen Digest's Senior Analyst, Piers Harding-Rolls.
Harding-Rolls notes that it is difficult to compare these latest PSN user figures to Microsoft's Xbox Live in a like-for-like type way because "these [PSN] registrations are not comparable to any announced Xbox Live subscriber numbers (as these are active accounts not lifetime registrations). The last reported Xbox Live subscriber numbers were 17 million at the end of 2008."
The analyst also adds the caveat that "neither of these numbers represent individual users (it's possible to hold more than one account as a user) nor individual consoles attached to the internet (it's possible to have more than one account on an individual bit of hardware)."
TechRadar requested future projections for both PSN and Xbox Live, though while Screen Digest has its own projections for both services they were not able to share them with us at this time."