About a third of the population claims to go to church regularly. The amount who actually do is somewhat less, surely. And anecdotally, amongst my churchgoing friends & family, especially the non-elderly ones, tithing isn't particularly common. It's pretty common with LDS, though, I understand.

I've never seen anyone do 10% of their income, I think that comes from the medieval period, or just wishful thinking. People usually drop a $10 or $20 in the plate whenever I've been.

It’s quite common in some evangelical denominations. Mormons do it as well.

Even when it’s not a formal tithe, American charity skews heavily towards giving to churches.