A simple test is if you got rid of a product, what would customers use as a direct replacement? Those competitor products, the product you got rid of plus the customers of those products, are a market.

Here, the products are Google Ad Exchange (Doubleclick Ad Exchange) and the Publisher Ad Server (DoubleClick for Publishers) which are now Google Ad Manager and the customers are publishers selling space for ads on their websites.

Website publishers can't really use Facebook as a direct replacement, so they're not in the same market.