You could have made the same criticism about a magazine in 1965. They tout to advertisers how many “readers” they have and how readership is growing, and your criticism is “all I do is open it and flip through the pages; I don’t _read_.”

Your value is not in reading the articles but in viewing the ads. For FB display advertising, you opened the app and viewed ads. Eyeballs captured! Value unlocked! Sure they’d rather have you spend 5 hours on the site, but even your slight attention is valuable.