Because social is/was such a fundamental aspect of the human experience. As proven by facebook’s continued explosive growth in the years since Google tried to compete. Not to mention FB’s ability to affect markets, attitudes, and politics at global scale.

It would have been better if they had competitors to keep them honest.

This was, in fact, the dominant narrative at the time. Niche search products like Amazon (products), Facebook (social), and Twitter (news) were going to be the end of Google if they didn't get their act together. It's the same reason they're so obsessed with becoming the destination or broker (ads) for search queries instead of sending people on to organic results.

Facebook is the only other company that has meaningful market share in online ads - the stats are always something like "Google + Facebook have 90-something %". Had Google managed to build a product that replaced Facebook, maybe they'd have that 90+% to themselves.

All large enough corporations expand until they can read email; those that don't are subsumed by ad-hoc implementations of 50% of Common Lisp. - Paul McCarthy (1956)

Facebook is perhaps the biggest competitor to YouTube with video.

can't search a closed Facebook network. Facebook sucked up the internet.