There are 4 possibilities for add friend button disappear:
1) The individual has changed his settings (Account settings) such that nobody can send a pal request, however that individual can send to others.
2) That individual has passed the amount of buddies and therefore can not receive any more pal requests. (In this situation, even they cannot send a pal request to you - to do that,they may should get rid of a few human beings from the present list)
3) Facebook has inserted some other function, to keep away from unknown human beings from sending pal requests again and again. If you've got sent a chum request to them as soon as (and in the event that they rejected and marked you as "junk mail"), then you would not be able to ship a friend request to them once more, but they may be capable of send a chum request to you.
4) You don't have any mutual friends, in that case you'll now not be furnished with the upload buddy option. In such cases, you'll be able to message them.