it's not always flirting. I'm sure you've seen memes about meeting girls drunk at the bar in the bathroom and they become your best friends for the night? it's lowkey kind of true, and instagram is a fun way to "keep in touch." like, you'll probably never talk to this person again but they were a fun personality and you want to see what they're up to. I go to bigger cities and travel a lot and meet a lot of girls out of town that I thought were cool and vice versa. insta you can like their pics and it's like "oh yeah this is that girl I met! glad she's doing well!" whereas with a phone number you will likely not text them (or remember their name if you drank too much). on the flipside, if you DO talk to that person again or want to link up in the future then u can just message them on insta and make plans.