Facebook says it's going to buy the mobile messaging company, WhatsApp, for around $16 billion.
450 million people use the service every month and the company is aiming for a billion customers. The app has more than one million new registered users each day.
It is understood WhatsApp will remain separate and the brand will be maintained.
Aidan Donnelly is from Davy Stockbrokers - he says it's a major development:
The company said in a statement that it would pay $4 billion in cash and $12 billion in Facebook shares as part of the deal.
In addition, the app's founders and employees will be granted $3 billion in restricted stock that will vest over four years after the deal closes.
The purchase marks the largest single acquisition in Facebook's 10-year history.
WhatsApp will "continue to operate independently and retain its brand" despite the acquisition, Facebook said.
"WhatsApp is on a path to connect one billion people. The services that reach that milestone are all incredibly valuable," Facebook CEO Mark Zuckerberg said.
The acquisition will also see WhatsApp CEO Jan Koum - a former Yahoo engineer - join Facebook's board of directors.
