Anecdotal, of course, but in my experience many people won't sign-up using their email because they're tired of getting added to mailing lists and receiving spam. Now, you could use fake emails when you sign up for these, partially as a way of avoiding spam and partially to find out which services are the most annoying with their mailing. However, this gets tiresome and if I can find an alternative service that doesn't require me to give as many personal details, I'll choose it over that one.