I hear some jobs do this and maybe colleges do the same. My guess is that they don't because it would be hard to track that down for everyone. Do you think an admissions official will look at my online accounts?
Not always, but there's a good chance that they might. It's best to assume that colleges will see your social media accounts. Try to avoid posting things that are offensive or portray you in a bad light - it's just not worth the risk.
Colleges have been surveyed several times in the past regarding their practices when it comes to researching candidates' social media presences. In this article, the researcher found that 86 percent of colleges surveyed had looked at prospective applicants' social media sites (granted, this was out of a pool of only 43 colleges, so it's not a great sample size). A slightly earlier survey conducted in 2013 asked 381 college admissions officers whether they had reviewed candidates' online presences. At that time 31 percent of them said yes, which represented a 5 percent increase from the results of the same survey the previous year. Assuming the numbers have continued to climb, it's probably safe to assume close to 50 percent of admissions officers are looking at social media sites.
Nowadays, it's easier than ever for colleges to check up on your internet presence. Keep in mind that what you post is out there for them (and anyone else) to see. Even if they don't point to illegal activities or bigotry, comments bashing other people or any other hurtful content in your posts could come back to bite you.
Tl;dr: Don't post that picture of you shotgunning a beer. You'll thank me later.