Popular messaging application WhatsApp on Thursday announced it now has 400 million monthly active users, a milestone it says no other mobile messaging service has reached."This isn't a count of people who just registered for WhatsApp — it's the number of people who are actively using the service every single month," the company said in a blog post. In the past four months alone, WhatsApp has added 100 million active users.The achievement is quite impressive, especially considering WhatsApp only has 50 employees, and most are engineers. Moreover, the company said it has not bought any targeted ads or marketing campaigns to promote the app."We're here because of all the people who share their WhatsApp stories with co-workers, friends, and loved ones — stories we love to hear," WhatsApp said.WhatsApp is available to owners of Android, iOS, BlackBerry, Nokia S40, Symbian, and Windows Phone devices. The 99-cent app allows users to exchange messages without a paid SMS plan, by using the same data plan on which emails and Web browsing run. The app's founders proudly stand behind the decision to not sell in-app ads.Meanwhile, the announcement comes just days after security firm F-Secure analyzed aparticularly nasty Android app that targets users of WhatsApp. Once the dangerous app is installed, it uploads your WhatsApp conversations to another website where anyone with your phone number can purchase copies.WhatsApp is available for download via various app stores, or through the startup's website. For more, see PCMag's full review of WhatsApp Messenger for Android and Windows Phone.