Both MySpace and Facebook each lost over one million users in September compared with the previous month, according to Nielsen Online.

The unique user for MySpace dropped by nearly three percent to 58.6 million in September versus the 60.3 million in August. That marked the first time since June that MySpace saw unique user figures at less than 60 million.

However, compared with the same period last year, MySpace seems to be maintaining its strong growth in audience. According to Nielsen, the site's unique user count for September represents a 24 percent increase versus the same month last year.

Similarly, while all reports point to Facebook with its meteoric growth – its audience in September increased by a whopping 133 percent versus the same month in 2006. However, its unique user slipped from 19.2 million in August to 18.1 million in September, a decline of six percent.

Most social networking sites experienced exponential growth from year to year as of September, but sites like Classmates.com (minus two million users) and the kid-targeted Club Penguin (minus 200,000 users) also recorded audience declines versus August, Media Week reported.