Welcome to the forum.
What is the pixel x pixel Image Size?
Can you post a link to the Web page, or the Image, showing the problem?
I would choose PNG myself, as JPEG compression can, well, not be so kind to text.
That is the page that shows it blurry
however this page http://i1263.photobucket.com/albums/ii621/inthehoney/3747799fcacopy.png shows it looking rather normal...idk..maybe it is facebook.
the pixels are 1920 x 1200
Problem is in Facebook not in Adobe, i removed cover from my pages because same problem, my covers have lot of text on it, I also tried PNG and JPEG to see if any difference.
Flickr is much the same, and does terrible things to your pictures when downsizing them.
Everywhere is the same when service must create thumbnail (downsizing): Facebook, Google, Flickr.. but Facebook shows blury images even when showing 100% image, when you upload image with exact size of image container. Google is better (Blogger, Picasa Web Albums, Google+) when showing images without downsizing them.Many photographers use Google+ to promote their photographs.