If it has, you probably pay to advertise on at least a few web sites. According to a recent interview with Google's spokesman Matt Cutts, Google has dramatically decreased the value of "paid-for links". Gone are the days when you could just throw tons of cash at your web site and shoot up in the rankings on Google.
Didn't pay for links you say? Still dropped in the rankings you say?
This is most likely a trickle down effect of web sites that do link to you and formerly ranked higher due to paid-for links.
Good luck trying to get people to link to your web site for free. :)