What eats the Prickly Pear Cactus?

All or most types of Galápagos Finches do.

When I was in Morocco at a souk (or market) a vendor was selling Prickly Pear fruit which he had piled in a pannier carried by a donkey. When we asked to buy he used a pocket knife to peel the spiny skins off the fruit. The flesh was juicy and sweet and a bit pippy. So to answer your question: people eat Prickly pear cacti.