[rbinv]

Isn't it somewhat problematic (with regards to privacy) to just send over users' email addresses this way?

Other than that, I like the idea. Good for site owners who can't code something like this themselves.

[empressplay]

Yes, anything that sends information in the URL is obviously visible to anyone who cares to look at it, and would breach most privacy legislation.

This is a good idea in principle but the execution isn't really viable in its current form. They need to re-think how they get the e-mail addresses and how they keep track of users.

[jnorthrop]

In addition they offer no privacy policy. That means they have complete freedom to aggregate all of the information you send to them, via your app, and sell it.