Oops.

Just three days after Facebook made serious waves in the tech and mobile world with its $19 billion acquisition of WhatsApp, a cross-platform mobile messaging company boasting over 450 million users per month, the app went down. The outage began early Saturday afternoon and, while the service has been restored in certain areas, continues to persist among some users as of approximately 6 p.m. ET.

Users of WhatsApp, a popular mode of international messaging in lieu of hefty text messaging fees, took to Twitter to vent their frustrations. Many made jokes, claiming the WhatsApp crew were too busy celebrating their billions, but most seemed to aim their ire at none other than Facebook co-founder, chairman, and CEO Mark Zuckerberg.

On Feb. 19, Facebook announced it had reached an agreement to acquire WhatsApp for $16 billion, including $4 billion in cash and $12 billion in Facebook shares, as well as an additional $3 billion in restricted stock. Under the watchful eye of Zuckerberg, Facebook’s policy seems to be “if you can’t beat ‘em, join ‘em.” The company has made a habit of squashing its competition by acquiring them in pricey deals, including its $1 billion acquisition of the popular photo-sharing service Instagram on April 12, 2012, as well as a $3 billion offer for Snapchat (which was rejected by the photo-messaging service).

But Zuckerberg, along with many other tech gurus, believes that mobile-messaging is the wave of the future and, while Facebook is the world’s premier mobile-messaging giant—with a reported 945 million monthly mobile users as of January—WhatsApp posed a serious threat, with 450 million monthly users and climbing (adding over 1 million new users a day). By acquiring WhatsApp, Facebook has cornered the mobile-messaging market in North America and Europe.

The tech giant will still have to compete with other mobile-messaging apps outside of North America and Europe, including LINE—a free mobile-messaging app that’s become huge in Japan and Southeast Asia, with over 300 million monthly users and revenue of 454.2 billion won ($423 million) in 2013; China’s WeChat, which boasts over 272 million monthly users, including 100 million abroad; and South Korea’s Kakao Talk, with 130 million monthly users and 5.5 billion messages exchanged per day.

UPDATE: The WhatsApp service was reportedly down from approximately 2:30 pm ET to just before 6:00 pm ET, but is now back online.