I dont know the challenges that others have faced but if i were to guess maybe certain websites begin blocking by domain name? It is hard to say, but its an interesting problem that i will have to address maybe as the product grows. At the moment I dont have an answer because it has yet to be a problem I have experienced personally.

Yes, I'm pretty sure it is by domain name.

One suggestion: custom domains for your users.

This could also help you "monetize", or at least help the service sustain itself.

Thank you for writing and sharing such a useful service.

People like you make the Internet better and offer hope for the future.