If you write about social media often, you might find yourself needing a few statistics. The following sources will give you the most current data available about social networking usage.
The following sites are a good place to start for statistics about most of your favorite social networks.
Wikipedia is a good first stop to get the latest stats about social networks. Their pages usually have each social networks launch date, current number of users, founders, revenue, traffic, and other information as applicable. You can see what I mean on Wikipedia pages for Facebook, Twitter, YouTube, Google+, and so forth. Be sure to click on the number next to each stat to see the official source for more information.
If you're looking for traffic and demographics, Quantcast is a great source. See some interesting information about Facebook, Twitter, YouTube, Pinterest, Google+, and so forth. Use the search box to find even more domains.
The Top 10
All About Facebook
Facebook seems to have the most data out and about on the web. Here are some great sources.
Facebook Fact Sheet
Facebook itself offers some basic statistics about their network on their company info fact sheet. it includes the current number of employees, number of monthly and daily active users, board member names, and locations of Facebook offices worldwide.
Inside Facebook Gold
Need some in-depth statistics about Facebook? Inside Facebook has a gold membership that allows you access to the latest demographics about Facebook usage. While their membership is currently closed to new enrollment, you can still get some interesting stats for free on their membership page including the number of users in the top 25 countries on Facebook and global audience demographics.
Facebook Pages Leaderboard
All About Twitter
Not quite as detailed as the information on Facebook, but the following are stats about Twitter.
What is Twitter
There aren't too many interesting stats on the What is Twitter page, but the ones that might catch your eye are the number of tweets per day and active users!
Top Twitter Users
TwitterCounter has a page for the most followed Twitter accounts. The top 20, with exception to Barack Obama, YouTube, and Twitter en espanol, are mostly celebrities.
All About LinkedIn
LinkedIn's About Us page covers the latest LinkedIn facts including overall membership plus breakdowns of the number of members in 14 different countries.
YouTube's Statistic page has tons of neat information including how many hours of video content is uploaded every minute, how many videos are viewed per day, unique users per month, user demographics, and more. There is even a section about social usage of YouTube such as the number of people connecting their accounts to other services (Facebook, Twitter, etc.), how many people auto share, and more.
What sites do you use to check social media statistics? Please share your resources in the comments!