What does a background check entail, and am I screwed?

I was offered a job at Toys R Us, and about four days ago they told me they were doing a background check... Haven't heard from them since, and I'm kind of worried.

I have no criminal history, but I have worked for a couple places very briefly (like, 2 days-1 week) and walked out because they were horrible, and I obviously don't put them on resumes. I've also been told businesses look through your social media... I've done nothing horrible, but recently had a discussion on my fb wall about how silly rules about hair color are at most jobs... Am I just really paranoid, or could they have dropped me because of something I've posted on the internet?

Job is at Toys R Us.