In public servers, Player.FollowUserId gets set correctly (from my experience at least). However, in private servers, the value does not get set (it remains at 0, the default value). This is a fairly small issue, but it would nevertheless be helpful if fixed.
Repro steps: from the website, follow someone into a game while they are in a private server that you have access to as well. Your FollowUserId will not get set.
Screenshots: Public Server vs. Private Server: