codyg1985’s Think Tank

After taking a look at the CSS style sheet for Facebook, the website does not use Segoe UI as the default font, but Lucida Grande, as evidenced by this code:

body, p {
font-size: 11px;
font-family: "lucida grande", tahoma, verdana, arial, sans-serif;
text-align: left; }

The fonts look very similar though. If you have Segoe UI on your system, you will able to see the two fonts:

This is Lucida Grande.
This is Segoe UI.

This is Lucida Grande at 13px.
This is Segoe UI at 13px.

As I said, the fonts look very similar. Note that if you don’t have Segoe UI on your system (this is most of you), Segoe UI will appear as Tahoma.