The interface is attractive enough and doesn't have any glaring usability issues.
Performance (on my Nexus S w/ 2G connection) could improve but isn't so bad that it causes frustration.
And it's available for free and without ads.
I get the style sheet bug up to a few times a week, but it's no big deal. Just pull the page down to refresh and it's fixed.
Maybe my standards are low but I suspect 95% + of people are the same, so why would Fb bother doing it any differently?