As others have mentioned, ISPs can game the speedtest.net results, whereas with fast.com users will get exactly what they want: a single number that shows how fast they get Netflix. No frills, nothing else. Some of the advanced features of speedtest.net are useful for some advanced users, but it's a weird de-facto standard that caters to the tiny minority.
Also the "powered by ookla" is the weirdest branding I've ever seen. Not only does it bother me, but makes me trust the site way less. I didn't know what ookla was until I googled it just now.
Redesign/rebrand speedtest.net so it matches ookla. Right now speedtest looks like a scam, ookla looks official.
Also this makes me feel weird: (all the links still go to c.speedtest but have google's content)
http://c.speedtest.net/javascript/speedtest-main.js?p=ltQVAd...