It's not surprising that Facebook serves its own ads, considering that it's one of the top 2 digital ad networks by revenue in the US. Google does the same for its sites.

Facebook and Google have a duopoly on the online ad market, with a combined 60.9% market share. Amazon is #3 with just 7.6%.

https://www.emarketer.com/content/facebook-google-duopoly-wo...