How can I flag (follow) user by REST API using service resources?
Try the following

Flag 

URL: http://example.com/apiauthd/flag/flag

Method: POST

Parameters:-
    flag_name: like
    action: flag
    uid: { uid of the logged in user}
    entity_id: { uid of the other user to be flagged}


Unflag

URL: http://example.com/apiauthd/flag/flag

Method: POST

Parameters:-
    flag_name: like
    action: unflag
    uid: { uid of the logged in user}
    entity_id: { uid of the other user to be unflagged}