Microsoft is buying the professional networking website LinkedIn for just over $26bn (£18bn) in cash.
The software giant will pay $196 a share - a premium of almost 50% to Friday's closing share price.
The deal will help Microsoft boost sales of its business and email software.
Microsoft said that LinkedIn would retain its "distinct brand, culture and independence".
Ben Wood, head of research at CCS Insight, said the deal would give Microsoft access to the world's biggest professional social network with more than 430 million members worldwide.
"That's a valuable asset that can be deeply integrated with a number of Microsoft assets such as Office 365, Exchange and Outlook. That said, Microsoft has stated that the company will continue to operate as an independent business, so we'll have to see how deeply the integration occurs," Mr Wood said.
Microsoft chief executive Satya Nadella said: "The LinkedIn team has grown a fantastic business centred on connecting the world's professionals. Together we can accelerate the growth of LinkedIn, as well as Microsoft Office 365 and Dynamics."
In an email to staff, he that the deal "is key to our bold ambition to reinvent productivity and business processes. Think about it: How people find jobs, build skills, sell, market and get work done and ultimately find success requires a connected professional world."
LinkedIn shares soared to $193.70 in morning trading in New York after the deal was announced.
Shares in the company, which floated in May 2011, have fallen by more than 40% this year.
The stock plunged by a quarter in February after the company issued a profit warning for the first quarter and reported an annual loss of $166m.
'Incredible opportunity'
Jeff Weiner will remain chief executive, reporting to Mr Nadella. He and Reid Hoffman - the chairman, co-founder and controlling shareholder of LinkedIn - both backed the deal.
"Today is a re-founding moment for LinkedIn," said Mr Hoffman. "I see incredible opportunity for our members and customers and look forward to supporting this new and combined business."
LinkedIn has been trying to expand by offering users more messaging options, mobile apps and a revamped "newsfeed" to help boost engagement.
Last year, the site pledged to send less frequent and "more relevant" messages after numerous user complaints.
The takeover is by far the biggest acquisition made by Microsoft, which paid $8.5bn for Skype in 2011 and bought Nokia's mobile phone business for $7.2bn in 2013.
The LinkedIn acquisition also eclipses the $19bn that Facebook paid for WhatsApp in 2014.
Despite having a cash pile of about $92bn, Microsoft said it would pay for LinkedIn mostly by issuing new debt.
It expects the deal to generate annual savings of $150m by 2018
