Microsoft first pulled back the link command in March of this year, due to trouble with automated queries. But as Karen reported, it seems to be back with a new prefix.

Just add a plus sign before the operators and it works. For example, a linkdomain command is now +linkdomain:www.searchengineland.com, and a standard link command is now +link:www.searchengineland.com.

Source : http://searchengineland.com/071113-083840.php