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.

Is Facebook or Twitter the better analogue to Snapchat? Twitter has revenues of around 600M per quarter but lost 80M as its growth stalls.

The difference is that Facebook was within reach of cashflow positive at all times in it's growth, ie if they fired non-core staff could be cash flow positive within months (source: Mark Zuckerburg talk I went to). It had ad's from within the first few months of existence, that was Saverin's role before he left/was fired but they always built that part of the business and revenues always scaled with expenses.