Facebook also had to pay $100,000 to buy the .com. That's not really a possibility for most of us.

I believe it was mymint.com

Not necessarily. I don't think SquareUp.com is horrific or anything if they never end up acquiring Square.com. It's not ideal but it doesn't have to be horrific either.

If the ideal name is four letters with no vowels then I'm not sure a few extra letters are that much worse

Even if the entire focus of my app was brevity I'd be very happy with tldrapp.com, tldrhub.com or sorrytldr.com (all available) as names :)