FB's competency is getting SMBs to advertise online. There are plenty of other social platforms (Reddit, Twitter, Snapchat, Tumblr), but they've all struggled to make money. The problem for emerging platforms is that Facebook spends $18B per year on R&D and hires many of the best engineers.
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Facebook has a teenager/young adult problem. Teens are always looking for their own space, so there is always whitespace for a new social media app. But that is a bigger problem for the social media company's that own the "teen" space. If it takes Snapchat >10 years to become profitable, but then TikTok steals their core audience, it is hard to make a good return on the billions invested in the company.