I hope they fix their typography too. Arial is not the way to go, especially when you allow paragraphs as posts (or essays). Line-height makes it even worse. </designer's-rant>
http://facebook.com (Lucida Grande is a fab choice)
http://readability.com - one of the key things this app changes is typography.
Check out default typography on thesis theme - http://64notes.com/thesis
Examples of bad typography:
google groups, Google Plus
I've many more if you want.