SALESFORCE.COM Inc, the biggest maker of online sales management tools, said it would pay US$2.5 billion for marketing software maker ExactTarget, which helps companies reach customers on social networks through mobile devices.

The acquisition - the biggest ever for Salesforce.com - is its eighth in the past year and its second big purchase focused on social media. It acquired Buddy Media, which helps big brands manage Facebook and Twitter pages, in August.

ExactTarget provides Internet-based marketing software used by companies to personalize e-mail and text messages and to run social media ad campaigns. It has 6,000 clients, including Coca-Cola Co, Gap Inc and Nike Inc.

But investors took a dim view of Salesforce.com's latest purchase, pushing down its shares 5 percent in early trading.

Big technology companies such as Salesforce.com and Oracle Corp are seeking growth through acquisitions as their traditional businesses slow and smaller rivals take advantage of social media.

Considered the leader in cloud computing, Salesforce.com posted lackluster quarterly earnings last month as costs rose following its buying spree.

Chief Executive Marc Benioff, who called the ExactTarget deal Salesforce.com's most important ever, said the company would now dramatically slow the pace of deal making.

The deal would cut Salesforce.com's operating cash flow by US$75 million to US$80 million in fiscal 2014.