Twitter's Vine six-second video sharing application has reached the 40 million registered user mark, up 27 million since June. Rival Facebook says its photographic acquisition, Instagram, has 130 million monthly active users but declines to state how many have tried the relatively new video-sharing function of the app. "What's clear is that the Twitter and Facebook units are duking it out, including courting brands, celebrities and artists to opt for their app," Shira Ovide writes.

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