I'm thinking of adding a share feature where a customer will have the option to select either to share a message on twitter/facebook/google+ or actually pay for an add-on service.
However, one issue I see with this is if the customer keeps on sharing to the same friends/family they have on social media they would be sending this to the same group of friends/family over and over again.
I see this being good because it's like a form of re-targeting, but I also don't want the same group to keep seeing the share message instead of targeting new users.
In my experience, yes. Dropbox did this with great success.