Long, long ago, when the net was flat and only geeks knew how to register domain names, a few savvy people started signing themselves up to own domain names like "www.mcdonalds.com," with the thought that, someday, McDonald's Corp. might want to be on the Internet and would offer them oodles of money to buy the name from them.
Eventually, it was determined that URLs could be subject to trademark infringement, and people who had registered domain names in hopes of earning a windfall had to give them up to the registered trademark holder. (Which also meant that there were problems like Virtual Works, Inc. v. Volkswagen of America, Inc., a dispute over the domain vw.net.)
This brings us to today, where the problem is "Twitter squatting." While this might sound like a bout with intestinal flu, in actuality it's someone registering a Twitter account, either to profit from a corporation wanting to buy the name, or by sending out information purportedly from the company. As far back as last fall, people were warning of the problem.
However, accounts that are set up for legitimate reasons -- such as tracking a company or providing a news feed about a company -- are acceptable, though they may be modified to reduce ambiguity. "Accounts created to help a community or provide information will be contacted with the appropriate steps to take to keep the account," Twitter says. "News feed accounts will more clearly designate that they are aggregating news about a company to resolve confusion."
Similarly, in terms of name squatting, "Attempts to sell or extort other forms of payment in exchange for user names will result in account suspension," Twitter says. In addition, "Accounts that are inactive for more than 6 months may be removed without further notice." Mass account creation is also a violation.
Facebook squatting is also an issue, but apparently not to Facebook; neither "squatting" nor "trademark" has an entry in the Help Center, and the "Avoid Squatters" page in Facebook is a service for occupying uninhabited London properties to keep people from squatting in them.