Facebook's WhatsApp messaging service, which is used by 1.2 billion people across the globe, is planning to move off of IBM's cloud and into Facebook's own data centers, according to a person familiar with the matter.

The WhatsApp move, which could begin later this year, would result in IBM losing a high profile customer for its public cloud. A source claims that WhatsApp has been one of IBM's top five public cloud customers in terms of revenue, and was at one point spending $2 million a month with IBM. (IBM says WhatsApp is not currently one of its top five public cloud customers.)

IBM's public cloud business lags behind Amazon Web Services (AWS), which is on top with 33 percent of the market in April, as well as Microsoft's Azure cloud, according to Synergy Research.

Facebook acquired WhatsApp for $19 billion in 2014 and has left the app running on the servers it has always used. Other companies that Facebook has acquired over the years initially used similar IBM SoftLayer servers, and Facebook was quick to bring them in-house, another source said. But with WhatsApp, Facebook has taken a much longer time.

Part of the reason was prioritization. When Facebook bought WhatsApp, it was already in the midst of a yearlong migration of its Instagram photo-sharing app, which it acquired in 2012, from AWS to its own data centers.

Also, Facebook separated its own Messenger messaging and calling app out from the core Facebook app months after the WhatsApp acquisition, and the social network's CEO, Mark Zuckerberg, didn't want WhatsApp to "get Facebooked" — in other words, to get integrated with its own apps too soon, a source said. Since then both Messenger and WhatsApp have gained hundreds of millions of users.

At IBM, WhatsApp is considered a poster child for its cloud, two sources said — just as AWS has the likes of Airbnb and Netflix to boast about, while Alphabet's Google points to Snap and Spotify. (Snap, which operates the Snapchat social media app, has committed to paying Google $2 billion and Amazon $1 billion for hosting over five years, the company disclosed in February.)