Handling contacts imported from users email account

Question

In my web application, I give the user the option to import all of his/her contacts from their email account, and then send out invites to all of these accounts or map the user to the existing accounts based on emails.

Now the question, is once all of these contacts are imported, would it be right to save these contacts back for repeated reminders, etc.?

I am quite confused here because that is the way all of the sites operate, but would that not be violation of data privacy? Is there an algorithm for this?

Answer 1

I think it would only be valid to store those contacts for repeated reminders people if they explicitly opt in to do so. Also very importantly, that reminder should not be sent unless the original user clicks on the magic button (s/he annoying their friends that's better than you annoying them).

Contacts for a user change all of the time anyhow, so inviting them to go through the process from scratch is probably better idea anyhow.