comScore Networks tracks Internet usage. They use a sampling approach "based on a massive, global cross-section of more than 2 million consumers who have given comScore explicit permission to confidentially capture their browsing and transaction behavior, including online and offline purchasing."
I don't subscribe to their service but I do follow their press releases. Around the middle of each month they give highlights of the last month's trends. Today's release gives data for the US-only for May.
- US Internet population 172.1 M
- MySpace unique visitors up 7% month over month to 51.4 M
- YouTube unique visitors almost doubles (in one month!) to 12.7 M
- Total visitors to all "community" sites they track were up 6% month over month
Whatever negative press MySpace has experienced, it doesn't seem to have slowed things down!