If you are looking for a free method then try the following -

Create a contest. Email your list of 100 fans and tell them they will get the following -

Their name in a special acknowledgments page in the book and a 50% off coupon for the price of the book if they share the link to your Amazon book page with one of their social networks.

Set a date for the contest to end maybe 2 weeks out. Whoever has the most shares across all of their social networks in that time frame (FB, Twitter, Instagram, LinkedIn, Blog, etc) will win a character cameo with their name in the book and a free copy of the book.

This would require writing in an ancillary character into a scene in the book that has no significance to the story but is just a way to mention the contest winner's name.

For each share, the contestants just need to email you the share link to prove they made the share.