PUT /myuser/following/theiruser

PUT /user/123 { id: 123, following: { foo, bar, baz } }

  PUT /user/123/following

    /user/foo
    /user/bar
    http://www.other-site-using-standard-formats.com/user/baz

Sigh, if only.

Yeah, if that's where your URL ended. But what if your URLs kept going? Say you had

    /user/123/followers

or

    /user/123/followers/followers

or

    /user/123/followers/followers/following

? When your URLs have a non-obvious terminus (as is typical of proper REST APIs) it becomes clear that the verb does not belong in the URL path.

So you're saying that we should have a verb instead?

FOLLOWERS_FOLLOWERS_FOLLOWING /user/123

No, the verb is FOLLOW, as in the example you were responding to:

    FOLLOW /user/123/followers/followers/following