I don't remember that. People thought that Facebook couldn't match Google-level ad revenue, but the monetization model was abundantly clear for Facebook as soon as it became obvious how much personal data they were tracking. Facebook IPO'd in 2012, at which point it already had over $4B in revenue, and had been generating revenue for years before that.

The much better example is Twitter, which people did indeed say wasn't monetizable, still isn't profitable, and has absolutely tanked since its IPO.