When Nielsen published "What Americans Do Online", it revealed that social networking and social gaming both beat out e-mail to dominate the time users spend online...on the desktop. However, it also revealed another interesting trend--when it comes to mobile online activity from devices like smartphones, e-mail is king.Mobile Web Use Now The Norm20 Mobile Trends and Future TechnologiesAccording to the Nielsen study measuring online activity of 200,000 users and comparing results from 2010 to the previous year, e-mail use on the desktop dropped from 12 percent to 8.3 percent, and fell into third place behind social gaming like Farmville. Meanwhile, time spent on e-mail on mobile devices went up from 37 percent to 42 percent--clearly dominating other mobile online activities.The trend highlights the increased reliance on mobile computing platforms like smartphones and tablets. A survey earlier this year from RingCentral found that 34 percent of business professionals conduct more business from the smartphone than from a desktop PC. It also revealed that more than 80 percent would give up coffee before they would surrender their smartphone.The good news for messaging platforms like Microsoft Exchange is that many smartphones--like the iPhone--are able to tap into the Exchange Server backend. However, the Nielsen survey may be bad news for desktop e-mail clients such as Microsoft Outlook, and increased reliance on mobile messaging could provide an opening for cloud-based messaging such as Google's Gmail to mount a greater challenge to Microsoft Exchange.Microsoft is hard at work putting the final polish on Windows Phone 7--Microsoft's own smartphone OS that comes with native Microsoft Office app functionality, including a mobile version of Outlook. Microsoft has also promised to work with partners to deliver a Windows 7-based answer to the Apple iPad and provide a more Microsoft-centric tablet experience.Assuming either of those platforms can capture 20 percent of its respective market, it will most likely be viewed as a significant success for Microsoft. The problem for Microsoft is that it still leaves the other 80 percent of the smartphone and tablet markets without native Microsoft Office and Outlook apps and opens the door for competitors.Microsoft has invested time and effort improving Outlook and incorporating innovations to integrate it with social networking. But, Microsoft should be concerned that the waning use of e-mail on the desktop and the increased reliance on mobile platforms for e-mail could eventually render Outlook irrelevant.Microsoft needs to see the mobile writing on the wall. The desktop won't disappear any time soon, but now is when Microsoft needs to be working proactively--and aggressively--to compete in the mobile computing world of tomorrow.Microsoft should be developing tools for all mobile platforms to ensure that--whether you use its smartphone and tablet platforms, or competing smartphone and tablet platforms--you continue to rely on Microsoft Office and Outlook.