It's been downhill for Gentoo Linux for some time now, http://www.google.com/trends?q=%22Gentoo+Linux%22
Even "linux" alone seems to have a downward trend by that measurement: http://www.google.com/trends?q=linux&ctab=0&geo=all&...
That's just my anecdotal evidence, Linux, when it is adopted by new people seem to run Ubuntu and aren't as much aware of other distros, I for one don't know much about Arc or Redhat or too much about how they differ.