Oct 28 (Reuters) - Software services company Zendesk Inc (ZEN.N) said on Thursday it would buy Momentive Global Inc (MNTV.O), owner of the popular SurveyMonkey platform for nearly $4 billion in an all-stock deal. Momentive shareholders will receive 0.225 shares of Zendesk"s stock for each share of Momentive stock they own, which implied a value of about $28 per share at the time of the announcement. Zendesk"s stock plummeted 16% in aftermarket trading, while Momentive also dropped nearly 9% to $22.70 per share. Momentive, then named Survey Monkey, went public in September 2018 at $12 per share. The San Mateo, California-based company rebranded itself to Momentive in an effort to expand to a portfolio of experience management services earlier this year.Zendesk, a customer service software vendor, has faced tailwinds during the pandemic as its enterprise customers accelerate digital transformation. This will be the company"s biggest acquisition."We have a big overlap in customers. It"s incredibly powerful. And we believe that it will create a whole new, richer picture of your customers," Mikkel Svane, chief executive at Zendesk told investors on Thursday. The deal is expected to close in the first half of 2022, pending approvals from Zendesk and Momentive shareholders. A sharp downturn in stock prices has previously resulted in the collapse of all-stock deals such as Zoom Video Communications Inc"s (ZM.O) $14.7 billion purchase of software firm Five9 Inc (FIVN.O). Zendesk also reported its quarterly revenue on Thursday, which grew 32% to $347 million. It expects the deal with Momentive to accelerate its revenue plan to $3.5 billion in 2024, one year ahead of its previous target.