Growth milestones are usually taken as an indicator of success.
A million users in a matter of days, 50,000 subscribers in your mailing list, the first million bucks before 30, all those Facebook fans and the fact your company made it to so many international markets so fast. All signs that things are well, so please carry on with what you're doing.
Alas, growth is a slippery slope if not accompanied by other equally important factors. We tend to obsess on what happens next and often what happens as we speak is neglected and often forgotten.
The problem with too much growth is that you typically lose connection strength, be it on the sustainability of your business, the trust your followers have in you, the community feeling around your brand and all that investor money which you can so quickly spend in the wrong things. And things such as these are hard to (re)attain, otherwise everyone would do it.
"Move fast, break things" is without a doubt a great motto to live by. Just make sure you're aware of the delicate balances you're dealing with and the consequences they carry.