Everyone except the people at Facebook seem to think the new font is rubbish! Here's a work around for Firefox and Chrome until its fixed. For Firefox, first install Grease Monkey (https://addons.mozilla.org/en-US/firefox/addon/748/), then install this script http://userscripts.org/scripts/show/89552 then refresh your Facebook page, simplez...
Facebook font-size changer for Greasemonkey
Reverse the recent font-size change in facebook