Still a little confused on the issue. Let me state as I understand it, and correct the missing pieces. You have Comcast where you are, and have two routers, Linksys and Dlink. Its fast wired and wireless, all PC's, all sites on linksys. On dlink, its fast on all sites on wired, but on wireless, only facebook.com is slow? If this is wrong, please correct.
If this is right, then I am not sure what to think. A RF issue with wireless wouldn't just cause one site to be a problem. Maybe Facebook is the only heavy site you go to? Have you tried like myspace, or youtube, or some other site that has alot of flash and javascript? Also, what is your general throughput on wireless? Have you tried tests like speedtest.net or visualware? And do you have more then 1 wireless devices? The fact that it's fast on VPN makes me think its a DNS or some AV issue, since the VPN would tunnel through all that and use your buisness's DNS. Maybe it has some item build on your wireless IPstack and not your wired? This is a good one. :-)