We have badges awarded to those who learned and used some site features, first of all Autobiographer and Informed. Scholar, Editor, Supporter, Citizen Patrol, Critic, Custodian also look worth mentioning as examples.
Propose to make a similar badge and award it for the first use of the "follow post" feature.
I have no strong opinion on the name for such a badge. "Follower" looks like a natural candidate.