Personally, we find 1.3 million registered users a strange number to start throwing all over the internet, but that's what's been going on so we may as well contribute. It's true -- 1.3 million users have subscribed to the PlayStation Network to-date. Approximately 600,000 of those are in the US. Among these users (the 1.3 million, not the 600,000), approximately 3.7 million things have been downloaded (no specifics, sadly).
For a newly formed system, that's not bad. Microsoft's XBox Live, which has been around since 2002, has an estimated 6 million users with over 135 million bits of downloaded content. Yep, Sony's got a ways to go to catch up, but if they've generated over a million users in 6 months, the number five years from now (using Microsoft's 2002 starting point, here) then who knows if that number will exceed the competition. It's good news, but it shows how Sony really does have a lot of work left to do and a lot of ground still to cover.