According to the figures published by Sony itself, it has 24 million registered accounts on the PlayStation Network as of E3 2009. That’s about a million more than actual consoles sold at the time – enabled by the fact that single consoles often have multiple accounts - which is a damn fair result all things considered. By comparison the Xbox 360 lays claim to 20 million active - an important word selection there by Microsoft - Xbox Live users, two-thirds of its 30 million install base: still a very respectable result.
Active is a key word. According to the E3 figures reported by the two companies, Sony gamers have downloaded 485 million bits of content, or roughly 20 pieces per user during its lifetime. By contrast Xbox 360 gamers have downloaded 1 billion bits of content, which equates to 50 individual bits of DLC per user. That’s a massive difference.
Perhaps even more painful for Sony is the fact that only 6 million of their 24 million registered PSN users, or 25%, have downloaded Home. That is downloaded, not necessarily using. This is a completely free experience available to anyone and it’s worrying fact that the vast majority of PlayStation Network users couldn’t be stuffed even checking it out, let alone actively participating. In its current form, it is clear that the PSN just isn’t working as well as it could be. So what’s wrong?
Read more here - http://www.gameplayer.com.au/gp_documents/PSNStruggle.aspx
It's an interesting read. It also talks about that fee Sony is charging to publishers for downloads and how it's affecting PSN's content.