The few days ago $500M round was just done because it was known Facebook was acquiring it. You can’t really count that. That was just VCs quickly doubling their money.
The other numbers are good, but then other companies also grow at similar rates. How come Instagram is the $100B valued company with 1B users and Whatsapp has over 1.5B users? And both are growing still. You’d think there would be another company coming at them.
For Whatsapp as a messenger the only service close to them is Messenger with around 1.4B users (you could say WeChat or Line/KakaoTalk as well though they are mostly popular in east Asia only). And Messenger is a sister product.
YouTube is a similar knock out success. Huge mindshare beyond its huge numbers, but it isn’t a profit machine like Instagram.
So it’s just Facebook that controls the 4 biggest social networks/messengers outside China. Along with some of them being the fastest growing services as well to this day.
All of this is to again say it wasn’t completely obvious Instagram would become this big. Other companies also grow and get valued highly quickly. And almost no other companies or services get as big as Instagram, which is still growing.
Finally, the zeitgeist at the time of the purchase was of shock at the price tag. Almost no one thought it was worth the billion dollar price.