When online user ratings site Yelp got its start ten years ago, it was all about soliciting feedback via email. The recommendation engine would encourage users who were searching for a
service to share their location and what they were looking for and then to share email contacts of other people in their area. Then Yelp would send an email to the provided email addresses and solicit
others in the area or who expressed similar interests. The company then used the responses to help build up its database. If they didn't respond, Yelp would keep emailing.