When you think about social networks, the first few that come to mind may be Facebook, MySpace and possibly Twitter. However, there are many different social networks out there with new users signing up daily. If you'd like to see how many of these sites a certain user is registered on, you can use what's called a "username checker." Keep in mind that this tactic will only work if the user registered on different social networking sites is using the same username.
Open your Web browser and visit a username-checking website. CheckUsernames.com, NameChk and KnowEm are sites that offer this service and operate in a similar manner.
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Type the username of the person you'd like to find into the box provided and click the "Check" button.
Read through the list of username results. Each website will either tell you that the username is "taken" or "available." If the website is "taken," it's very possible that the same user registered on each of those sites with the same username. You can then count and figure out how many websites they're registered on.