This is a valid argument but it does not answer my question which was about why do companies have developer advocates for products other that what they directly sell.
Because those products form an ecosystem on which the product is built which they do sell. If the users are familiar with the platform and have adopted it, it's easier to sell to them stuff based on the platform.
i am pretty certain that promoting react has little impact on people who do bring the most revenue to facebook (end users who simply eat up clickbait).