I do find it weird when I don't know their surnames, but I've never asked, because no one's asked me.
I'm going to an event with one guy next week, where we have to show ID, so I will need to give him my name, but I think he already knows it, because we work in a similar field, and a comment he made means I suspect he knows one of my colleagues, but I decided I didn't want to know for sure. Also,he might have seen something with my name on. I found him on LinkedIn, so I do know his surname, and LinkedIn sometimes tells you who's looked at your profile, so he might know I know, too.
It is weird, though, that these days, you might have exchanged phone numbers, but not real names, as people don't always sign up to dating sites with their real name. Mind you, back in the '90s, one of my housemates never gave her real name to guys on first meeting, so it's just different media, OLD app vs pen and scrap of paper.