Last Monday, Compete.com released the January ’09 numbers for the top 25 social networks .
I decided to have a closer look at those numbers and compare it with the numbers from another audience measurement company: Quantcast 
As you can see from the two images below, there is a big difference in the numbers from Quantcast and Compete. Compete.com uses a U.S. based panel for its reach estimates. Quantcast uses an inference-based, direct measurement methodology . Quantcast also offers a direct measurement solution for web site publishers.
I’ve been using Quantcast for about 18 months to measure my own web site performance and reach on a U.S. and global level. My Quantcast profile can be seen here: http://www.quantcast.com/p-6btf0Pyh9xonk 
You can click on the images for a larger view…
The last column in those images shows the rank by the other measurement company.
Compete.com Top 25 Social Networks – Jan ’09 (credit Compete.com)
If you’d like to get the Excel file with all the numbers, then please click here .