[eli]

Because it's a pain to set up a new Facebook profile. Presumably enough of a pain that people won't be willing to do it every 3 months to avoid paying for BrowserStack.

[dmethvin]

I'd assume that as well. I have collected several Microsoft IDs by way of Hotmail/live.com/outlook.com but I only have one Facebook account.

[stephenr]

Not that much of a pain if you don't need to develop apps with it.