Shares of Slack rose as much as 32% Wednesday following a Wall Street Journal report that Salesforce held talks to buy the company.Salesforce stock dropped around 4% on the news.The deal would likely value Slack at more than its market cap of $17 billion, according to the report. However, Slack’s market cap ballooned to more than $20 billion following the WSJ report.Salesforce has been on an acquisition spree, taking advantage of its market cap growth in recent years to buy growing companies. It acquired MuleSoft for $6.5 billion in 2018, the company’s biggest deal ever at the time, to help connect cloud applications. The following year it spent more than twice that amount on Tableau, acquiring the data visualization company for $15.3 billion.Slack would be one of the biggest software deals ever for the industry. It would rank among IBM’s $34 billion purchase of Red Hat in 2019, Microsoft’s $27 billion purchase of LinkedIn in 2016 and Facebook’s $19 billion purchase of WhatsApp in 2014.”This would be a game changer move for Benioff & Co. to further build out its collaboration engine and product footprint as cloud spending ramps across the enterprise,” Wedbush Securities’ Dan Ives told CNBC, referring to Salesforce CEO Marc Benioff.A deal is not guaranteed and the talks could fall through, the Wall Street Journal reported.Shares of Microsoft, which competes with Slack through its Teams offering, also dipped slightly on the report.Microsoft already competes with Salesforce in software for tracking customers. Microsoft reportedly looked at buying Salesforce but no deal came about. Later, Microsoft launched the Teams communication app.”Our primary competitor is currently Microsoft Corporation,” Slack said when it sought to become a public company in 2019. Microsoft has a large customer base it has been able to convert to Teams, and Salesforce could give Slack a similar benefit.Slack last said in October 2019 that it had more than 12 million daily active users.Spokespeople for Salesforce and Slack were not immediately available to comment.– CNBC’s Ari Levy and Jordan Novet contributed to this report.This is a developing story. Please check back for updates.Subscribe to CNBC on YouTube.