If you watched The Social Network, then you'll know that one of the early uses of Facebook was as a platform for measuring the attractiveness of other people.
It then spawned into a cultural phenomenon where pretty much everyone uses it, whether for networking, staying in touch with family, or connecting with internet communities, all for free.
The problem with free services is that, like most free things, it isn't exactly free. While Zuckerberg explains that Facebook doesn't directly sell your data to advertisers, Facebook uses the information users provide to target ads to specific audiences. Once advertisers tell Facebook the sort of user they want to target, Facebook can place the ads on accounts that are the right fit.
The implications of this data use are being thoroughly covered, so there are plenty of places you can check out the nitty gritty details that may teach you some things you didn't know about the way Facebook works.
Even better than that, though, are the jokes and memes to come out of the hearing.