Are there any "successful" startups that use PC's anymore?

I think its the startup equivilent of "no one has ever gotten fired for buying IBM".

When you need a developer machine, and don't have the time or care to research the build quality, configurations, etcetera of various PCs in order to maximize value then you simply buy a Mac.

You pay a premium, but get a machine that will at least be adequate.

I get the strong feeling that Stack Exchange is PC-heavy. They seem to be very much in the Microsoft ecosystem.

Everyone knows PCs are the superior machines. However, Macs get things done, in a very simple elegant way. Also, a Mac is needed for iOS app development. Macs can also be used for Android development. Win, win. Most start ups are based on mobile app development now days.

Yes. Of the programmers I know, the split is probably 60/30/10 PC/Mac/Linux.

The programmers I know who use Macs tend to be typical Mac users who happen to program, not people who chose the Mac because it was the best programming device.