Slack's CEO just tweeted a number that shows his $2.8 billion startup is still growing like crazy

Slack, last valued at $2.8 billion, is often called the fastest-growing business app ever. Now it seems like it's growing at an even faster clip, according to a tweet by its CEO Stewart Butterfield.

"20% of all of Slack's daily active users - one in five - started using it since New Year's," Butterfield tweeted Wednesday.

To put that in context, Slack said it had 2.3 million daily active users just a couple weeks ago, when it shared its latest numbers to celebrate its two year anniversary. That means roughly 460,000 (20%) of the 2.3 million daily active users are people who just started using Slack in the past two months.

And given that Slack had 520,000 daily active users by its first year anniversary in February 2015, it means the work messaging app has added roughly the same number of new daily active users in the last 2 months as it did in the first 12 months of its launch.

Moreover, if Slack can keep up this pace, it'll add roughly another 2.7 million daily active users by the end of the year, far outpacing what it did in its first 2 years.

Of course, it's almost impossible to keep up the same rate of growth for the whole year, and plenty of things could happen to slow its growth. For instance, Slack saw huge dips in its daily active users right around the holiday season for two years in a row.

But it's a reflection that Slack is still growing like crazy, even after two years since its launch, and it'll likely continue to be called the fastest growing business app ever.

Here's an infographic Slack shared on its two year anniversary: