WhatsApp reaches 600 million active users

FACEBOOK'S investment in WhatsApp is looking more and more promising.

WhatsApp now has more than 600 million monthly active users, according to a tweet from CEO and cofounder Jan Koum late Sunday night.

That's up from the roughly 450 million it had in February when Facebook first announced plans to acquire the messaging service for $16 billion with another $3 billion in restricted stock units for employees kicked in for good measure.

WhatsApp previously announced hitting the 500 million active user mark in April, meaning it has added 100 million active users in just four months.

Facebook CEO and cofounder Mark Zuckerberg alluded to the staggering growth rate in the original announcement of the acquisition, leading some to conclude that WhatsApp may be on pace to hit the 1 billion user mark more quickly than Facebook did.

With 600 million users, WhatsApp continues to be significantly ahead of its competitors in the fast-growing messaging app space.

Free messaging and calling app WeChat revealed earlier this month that it had 438 million active users, and analysts estimated last month that Line had around 235 million active users.

The WhatsApp acquisition received regulatory approval in the U.S., but it is still being reviewed in Europe. In its most recent earnings call, Facebook's execs reiterated that they expect the deal to close later this year.