Amid Mobile Effort, LinkedIn Falls ShortRead about LinkedIn's disappointing second-quarter forecast, Google's plans for your app activity data, Warren Buffett's new online presence, and more social-media news.

This week's need-to-know social-media news.

LinkedIn's stock took a tumble on Thursday after company executives said in a conference call that it expects second-quarter revenue to fall below analyst estimates. The company is forecasting revenue of between $342 million and $347 million, as opposed to the average previous analyst estimate of $359.3 million. LinkedIn is expected to report Q2 earnings in August. The news caused LinkedIn's stock to drop 10 percent, to $181 per share. Nevertheless, unlike other social media companies such as Facebook and Zynga, LinkedIn outperformed Wall Street expectations in the previous eight quarters. Its first-quarter revenue this year was nearly $325 million, far greater than the $189 million the company earned in the first quarter of 2012.

LinkedIn rolled out mobile ads for the first time last month, hoping to reach the increasing number of users who are accessing the site with their phones and tablets. But the company said Thursday that its advertising business will experience moderate, rather than rapid, growth. "We're seeing some encouraging early signs, but it's off a very small scale right now," said LinkedIn's chief financial officer Steve Sordello.--Reuters

Google integrates social activity from apps into search results.
Google is beefing up its search engine results with user data collected from mobile apps, Seth Sternberg, product management director for Google+, said Tuesday at the TechCrunch Disrupt conference in New York City. Since February, Google+ users have been able to use their Google+ accounts to sign into apps such as music player SoundCloud and movie site Fandango. Google has been collecting data about user activity on these apps, and in a few weeks will begin displaying this data in search results. For instance, when you search for Fandango, Google will now display the most popular movies of the moment based on activity in the Fandango app. "We're making it easy to discover what other people are doing around the Internet when they're doing a search on Google," said Sternberg.--CNET

Twitter's Vine app adds new video feature.
Twitter's Vine app for iOS -- which allows you to record six seconds of video -- released an update this week allowing you to record video with the iPhone's user-facing camera. Previously, you could only shoot video with the camera facing away from you. Using the app, business owners could deliver brief "face-to-face" messages to their customers, or give fans a peek behind the scenes of the business's operations. People have already been putting the app to some creative uses. In February, a journalistused Vineto create a playful "resume" while looking for a job. No doubt there are many more uses waiting to be thought of by entrepreneurs and consumers alike.--AllTwitter

YouTube users now watch six billion hours of video per month.
Google announced that its mammoth video service YouTube now streams six billion hours of video each month. In ablog post, the company pointed out that this usage totals "almost an hour [of video] a month for every person on Earth." Last May, YouTube users were watching around three billion videos per month, the company said. As YouTube continues to enjoy explosive growth and launches original short films and web series built around Hollywood stars -- such as last year'sSerena, featuring Alfred Molina and Jennifer Garner -- the platform becomes increasingly attractive to advertisers.--GigaOm

Warren Buffett joins Twitter.
Warren Buffett, the billionaire chairman and chief executive of investment firm Berkshire Hathaway, opened aTwitter帐户on Thursday. His first tweet -- "Warren is in the house" -- has been retweeted over 33,000 times. Within 24 hours, he had amassed nearly 290,000 followers after only two tweets. His second tweet included a link to a recent essay he wrote for Fortune magazine about the importance of women to America's economy.--SocialTimes