Trademark law exists to prevent others from doing business under your registered name, but before you can challenge them in court, you have to be able to catch them. And while people are far less anonymous than they believe on the Net, it can still take time to track infringers down.
First we had cybersquatting: the greedy and foresighted would buy domain names like CocaCola.com long before the company that had trademarked that name even thought of going online, then charged the company an exorbitant amount to buy the domain from them.
And then there was phishing, the act of luring consumers to sites that look like eBay, PayPal, or their bank, and asking for all their login information, including to Social Security number and mother's maiden name.
Now, with new social networks and Web 2.0 services springing up every day, there are more and more places that a company either needs to be or to monitor in order to protect itself from "brandjacking."
The latest victim of this form of brand impersonation is ExxonMobil. Due to the high-profile coverage of Comcast's presence on the microblogging platform Twitter, the creation of a Twitter account with the handle "ExxonMobilCorp" was more plausible than it would have been six months ago, even if some of the 140-character messages sent by "Janet" seemed off key.
If you've taken the trouble to trademark a company name or product name and build up a brand, then it's worth making sure that you're the one to put that name on the social media map and register as a user with the social networks.
Even if you never use the account, at least no one else will be able to abuse your good name.