Could Instagram be bigger than Facebook someday? If it keeps growing the way it has been, it's only a matter of time.

On Wednesday, co-founder Kevin Systrom revealed that his photo-sharing service's worldwide user base has reached 300 million. It was only nine months ago he was announcing it had surpassed 200 million.

With that kind of size and growth rate, if Instagram were a standalone company whose users were valued comparably to those of competing services, it would likely be worth more than $20 billion -- perhaps even as much as Uber, whose latest round of funding valued it at $40 billion. 

The new milestone means Instagram is for the first time officially bigger than Twitter, which claimed 284 million monthly active users in its latest quarterly earnings report. But Twitter has struggled mightily to maintain the growth pace of its early years; lately it's been onboarding new users at a clip of only about 5 percent per quarter. The sluggishness has forced CEO Dick Costolo to adopt a new strategy of emphasizing the total reach of Twitter's content rather than the number of people actively using the platform.

Instagram is also outrunning Snapchat, probably its biggest rival for primacy in the realm of sharing photos on mobile devices. It took Snapchat more than three years to rack up its first 100 million users, hitting that total this past August