Hmm in a world full of strange people on there, it is hard to pin point just one instance to call the strangest. Ill go with top 2

1. I met someone who had like a whole family on there right, which in itself is not all that strange or uncommon, the strange part was that each member of the family was just a different one of her accounts. So she would like post a pic and then comment from the other accounts. Multiple devices running , in a room with yourself your kid, your husband, and all are actually just yourself. hahaha. still cant wrap my brain around that one.

2. I had a girl I met a while back completely fall in love with me. Changed profile name, whole thing right. Would message me every second of the day, which is not the strange part. The strange part was that I found out it was someone i used to hang around with, trying to literally troll me. Wasted about a week or so on that garbage hahaha.
As some mentioned above, i now try to do a bit of research before accepting friend requests or talking lol.