Today I received this comment on my blog from a Spock employee,

Hi,

This is Maia, from Spock. We apologize for this mix-up. As part of our sign-up process, there is a checkbox to invite all of your contacts to Spock. After the checkbox, there is a pop-up that ensures that you want to invite all your contacts. Our logs show that when you signed up, the checkbox was selected to invite all of your contacts. It’s possible you overlooked this checkbox and pop-up, opting in to invite your contacts.

We are not trying to spam your contacts, as we thought we had your permission. Your contacts will not be e-mailed again.

Again, we apologize for this inconvenience. We’re working to make sure this doesn’t happen again, perhaps as a user setting as you suggested. We never e-mail your contacts without your authorization. Because of your comment, we are going to make this feature more apparent in the UI. Our engineers are already working on the change.

Best,
Maia

I am pleased that my blog caught Spock’s attention and that they are going to fix this feature, as I found it to be very misleading. Hopefully this benefits other Spock users as well.