WhatsApp has gained 100 million monthly active users (MAU) over the past five months, hitting a total of 900 million MAU, founder Jan Koum announced in a Facebook post.

It was only in April that the Facebook-owned instant messaging service, which has also launched a web version and voice calls for its Android and iOS users, hit the 800 million MAU milestone.

The $22 billion WhatsApp acquisition by Facebook last year was still impacting the social networking giant’s financial results in Q1 2015 when revenue fell short of analyst expectations. CFO David Wehner explained that “stock-based compensation and amortisation expenses related to the WhatsApp acquisition contributed significantly to the year-over-year growth in GAAP expenses.”

Although no monetisation strategy has been apparent with WhatsApp, in May it was reported that Facebook may finally have a plan to make money by letting businesses pay to contact users of the messaging app.

Facebook was said to be already testing such a service for its own Messenger app, which recently reached 700 MAU, and it may apply this to WhatsApp as well.

Facebook itself boasts 1.49 billion MAU (as of end-June 2015). 1.31 billion are mobile MAU.