Most admissions offices don't systematically search through applicants' social media, but they will absolutely look if something gets flagged to them - like if you tag the school, or if your public profile shows something really problematic. The key thing is that they're not hiring private investigators; they're overworked and processing thousands of applications. What they *do* care about is what you put in your actual application, essays, and what teachers say about you.
That said, if your stuff is genuinely "pretty normal," you're probably fine. We're talking about the difference between posting memes and normal life updates versus, like, bragging about cheating on exams or doing something illegal. Making your accounts private isn't a bad idea anyway for privacy reasons, but don't freak out and start deleting everything - that honestly looks more suspicious if an admissions person does poke around and sees a totally blank account. Just use common sense: would this embarrass you if your grandma or a college professor saw it? If yes, maybe reconsider posting it. If it's just you having fun with friends, you're good.
The real energy should go toward your application itself. Your essays, grades, and what you actually tell the school about yourself matter way more than whether your Instagram has thirty photos of you at parties or whatever.