NEW YORK (TheStreet) -- When Yahoo! (YHOO) announced it was buying Tumblr for $1.1 billion, many, including myself, saw it as an expensive acquisition. It looks like it may wind up paying off, however.

Speaking at the Morgan Stanley Technology, Media and Telecom Conference, CFO Ken Goldman said he was "pleasantly surprised" at how often Tumblr came up in the conversation with Yahoo! and Tumblr advertising sales force. "The re-blogging capability of Tumblr is phenomenal," Goldman said. "It's very successful."

On Yahoo!'s fourth-quarter earnings call, CEO Marissa Mayer said the average sponsored post on Tumblr for a native ad is reblogged more than 10,000 times.

Though Goldman did not give specific metrics, he did note that since the acquisition, Tumblr's sales force has increased its knowledge base of the advertising space, "knowledge they didn't have prior to the acquisition." So far, Goldman is "very pleased" with the engagement on Tumblr, noting companies, such as Apple (AAPL) - Get Report, are now using the platform to advertise their products and wares. Apple recently started advertising its iPhone 5c using Tumblr.

On the earnings call, Mayer noted that Tumblr has seen an increase in time spent, rising 50% year-over-year, and the mobile posts are soaring. During the same timeframe, mobile posts rose 200% year-over-year, with total posts growing 40% over the same time frame.

Goldman did not provide any update on when Tumblr would become accretive to Yahoo!'s earnings. However, CEO Marissa Mayer has previously said  "We said when we first acquired Tumblr that we would have it be accretive to EBITDA probably in 2014."

-- Written by Chris Ciaccia in New York

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