Nearly three-quarters of Facebook's advertising revenue now comes from mobile ads, as many more users access the social network on smartphones and other hand-held gadgets. On that front, the company is doing fine better, even, than it has in previous quarters. But the rate of its overall revenue growth slowed in the first three months of this year, causing its high-flying stock to tumble after the results came out. Wednesday marked the first time since early 2013 that Facebook failed to top Wall Street's expectations with its results. The social network had 1.44 billion monthly active users as of March, up 13 percent from a year earlier. The number of users who accessed Facebook on mobile devices at least once a month grew 24 percent to 1.25 billion.Revenue increased 46 percent to $3.54 billion, from $2.5 billion a year earlier. Analysts had expected $3.56 billion.