It's a well trodden hypothesis that Facebook optimizes for similarities, but I don't know if it's that straightforward.
Something that might be worth considering is what kinds of interactions are enabled by Facebook for any given person—who they are between, what the substance of the interactions are, etc. For example:
- I communicate with my friends, family, and extended network of people I've met over the course of my life (college, work, travel, projects, etc.) with my profile, News Feed, and Messenger
- I buy and sell random products on Marketplace
- I engage with communities of people over shared interests, identity, causes, initiatives, etc. on Groups, with Fundraisers
- I read news and updates from publishers I subscribe to on Pages
- (This list isn't exhaustive, but you see my point...)
In a way, it's a wide ranging social infrastructure—like a digital city. In some of those interactions, you meet people who are similar to you (and you _want_ that). In others, you meet people who you wouldn't ever imagine yourself communicating with, but you've found a context that you share some kind of edge that brings you together. By exposing yourself to people different from yourself on vectors that actually bring you together, you also expose yourself to their perspectives, worldviews, and lives.
Of course, I'm not naive into believing that every person's experience on Facebook mirrors this diverse, multi-faceted exposure to people both similar and different from oneself. For some people, the experience might just be a big reflection and validation of their worst instincts, but this isn't actually as frequent as people think. There _are_ ways to build product that enable people to find an experience that is expansive to their lives instead of constraining of it, and as much as it may not seem to be the case, I do think Facebook consistently moves in that direction as it continues to grow and evolve.