How many of your users come from "powered by" marketing?

I've been reading a lot of founder interviews and one of the main acquisition channels a lot of them mention is having some version of a "powered by" link on their customers' websites. The most obvious example is live chat. Then widgets (one founder noticed that after a local newspaper started using their widgets, almost every competitor signed up). Have you noticed something similar with your startup/product? What % of your paid users come from seeing that their competitors are using you?