Simply because a trademark owner hasn’t used his trademark in a domain name, Facebook profile name, Twitter name, or other social media does not mean that you can grab the name and use it to drive traffic to your business. This is the same trademark infringement issue one sees where a business uses its competitor’s trademark as a keyword in a Google AdWords campaign.
It isn’t a gray area. You can be held liable for infringement, deceptive practices, and under several other legal theories. If the trademark is registered with the U.S. Patent and Trademark Office, you might also be on the hook for hefty statutory damages plus the trademark owner’s attorney fees.
When in doubt, don’t use a competitor’s mark in your marketing and advertising until you’ve consulted with a lawyer who knows intellectual property law.