You can think of the mapping between a user and a thing to follow as a unique resource in of itself, which can have two different states, either true or false depending on whether the user is following the thing in question.
So a way of doing this would be for the client to PUT a new state of that resource on the server, since the client should already know the location of the resource. Your URL schema could be something like
or something similar that conveys that this is a resource representing the state of mapping between user and thing.
A GET on this resource would tell you if User A is following Thing B
And the client can change that state by
PUTing a new state
204 No Content