Facebook is having a run of milestones lately.A billion people used the social network in one day at the end of August and now its WhatsApp messaging app has hit 900 million monthly active users.

Facebook bought the app at the start of 2014 and during that time the userbase has more than doubled, with 100 million people joining the service in the past four months alone.

The milestone was announced by WhatsApp’s co-founder Jan Koum on his Facebook page. Facebook’s founder Mark Zuckerberg was among the first to congratulate him and even posted a photo of Mr Koum as he posted the status update.  By April this year, this had risen to 800 million and within the past four months the service has added an extra 100 million. 

However, WhatsApp has been criticised for not having a viable business model to take advantage of this growing userbase. The app’s founders, for example, take a strong stance against adverts. As a result, the app is free to download on all handsets but costs costs $0.99 (69p in the UK) for a lifetime subscription after the first year.

If all of its 900 million users eventually ended up purchasing a subscription, WhatsApp would make $891 million (£583 million). Following the acquisition, Facebook separated its Messenger service from the social network, much to the dismay of many users. 

To use the service, Facebook members now have to download a separate app, and figures suggest it has around 700 million active users a month. This means Facebook dominates the messaging app market, eclipsing the likes of WeChat and Viber, which have 600 million and 236 million monthly active users respectively. 

Facebook announced it had hit the one billion users logging on in a single day mark at the end of last month. The social network is now so popular, it accounts for a fifth of the time we spend on our smartphones, with many people logging in dozens of times a day, found a recent study.

Mr Zuckerberg went on to say: ‘I’m so proud of our community for the progress we’ve made. ‘Our community stands for giving every person a voice, for promoting understanding and for including everyone in the opportunities of our modern world. ‘A more open and connected world is a better world. ‘It brings stronger relationships with those you love, a stronger economy with more opportunities, and a stronger society that reflects all of our values.

‘Thank you for being part of our community and for everything you’ve done to help us reach this milestone. ‘I’m looking forward to seeing what we accomplish together.’ In July, the firm revealed new figures for its mobile apps.

It now has almost 1.5 billion users around the world who check their Facebook page at least once a month - the equivalent to the populations of China and Nigeria combined. That means that half of the three billion people who have access to the internet worldwide, now check the social network on at least monthly basis.

However, many of those are doing so much more frequently than that: around two thirds of users log in on a daily basis, the company said as it announced its quarterly results in July. The enormous figures helped to drive Facebook’s revenues up by nearly 40 per cent in the three months to the end of June, as it sold more and more advertising to the companies that want to reach its growing pool of users.