(Reuters) - Web-based software maker Salesforce.com Inc said it would buy marketing software provider ExactTarget for $2.5 billion as it looks to build a marketing platform that will tap increasing use of mobile devices and social networks.

The offer price of $33.75 for each ExactTarget share represents a 53 percent premium to the stock's closing on Monday on the New York Stock Exchange.

ExactTarget shares rose 53 percent in premarket trading on Tuesday, while Salesforce.com's shares fell 3 percent to $39.85.

ExactTarget, Salesforce.com's eighth acquisition in the past year, has 6,000 customers including Coca-Cola Co, Gap Inc and Nike Inc.

ExactTarget provides Web-based software for online marketing.

Salesforce.com said it expects the deal, which will add $120 million to $125 million in revenue in fiscal 2014, to close late in the second quarter ending July 31.

The acquisition is expected to reduce fiscal 2014 earnings by about 16 cents per share, Salesforce.com said.

(Reporting by Sayantani Ghosh in Bangalore; Editing by Saumyadeb Chakrabarty)