Mark Zuckerberg, founder and CEO of Facebook, is a named inventor on a Facebook patent filed way back in 2006. The patent, US8225376B2, relates to your Facebook privacy settings and how you, as a user, can control what different groups can see.
When you select a privacy setting, Facebook gives you a summary as to which other users can see your information, and what categories of your information (e.g., photos, email info, status updates, like activity, etc.) are visible to different groups (e.g., everyone, friends of friends, friends, or no one). This way Facebook (and Mark) provides you with more control over who sees what.
Although this all may seem obvious and second nature to us now, this patent was filed at a time when social media was in its infancy. This feature, by giving us more control over our privacy, may have helped people feel more comfortable with sharing information online which may have helped the widespread adoption of Facebook.