Growing ad revenue 30% YoY with users rising 8% YoY means FB is really improving on monetizing its userbase.
https://techcrunch.com/2019/02/20/emarketer-digital-ad-forec...
Nearly 19% increase in online ad spending overall + FB's 8% growth + improved monetization and you can see why FB ad rev increased 30%. Online ad spending is expect increase by double digits for many years so FB, GOOG, etc are sitting pretty as long as they can hold onto and grow their userbase.
Even if FB did nothing, their ad revenue would most likely increase by double digits every year simply because the digital ad space is increasing by double digits every year. Money is pouring into digital space and they have one of the biggest buckets.
Also much of the increase in online ad spend comes because of the improved monetisation of online ads. If Facebook had just sat on their hands, I'd estimate that their revenue would have grown 4% due to user growth, 3% due to growth in average user income, and 5% due to increase in average screen time. That's a 12% tailwind.
Facebook's actual revenue grew 26% so it's reasonable to say that around 14% of that was due to Facebook improving their ability to match users with advertisers.
(Disclaimer: I own FB shares)
Generally for a service like FB, just by becoming more valuable and useful to its users revenue can increase non-linearly with increases in users.