Is it normal he wont add me on facebook

Im seeing this guy and its not been long but we are talking on messenger so I added him on facebook. When he hadnt accepted or declined for 2 days I mentioned it to him and when he replied back he said "oh I must've missed that as i'm rarely on there but i'll accept in the future" then he changed the topic. I find this strange. Why would he not go and accept right away? Or even later? ...
You hear a lot of horror stories so even though he seems sweet it makes me suspicious. His relationship status is not set to public after all so I can't see it.
If he had a girlfriend though... he'd probably not be dating me openly where we can run into anyone, but it's still a suspicion I have now.
Most guys i've known barely uses facebook but no one has reacted in such an avoiding way ever.
It's also the fact that he added someone on facebook yesterday so either he's lying about not using it and not having seen my request, or it was someone he's added in the past who didn't accept his request until yesterday.
It's impossible to know and i'm not gonna ask. It's an observation I happened to make when I checked if he had accepted my request.

Should I be cautious?