Happened to notice today that link units hadn't been performing as well as they usually had on my site -- they were once the second-best performing ad unit for my site. Now they've fallen to 4th or 5th and sometimes 6th best, and when I compared March 2012 to March 2011, the difference was pretty dramatic -- and not in a good way.
Any of you guys experiencing the same thing with link units? Do you use them?