The No. 1 activity people do with their smartphones is email -- LinkedIn says that up to 28 percent of a typical professional's day can be gobbled up by reading and responding to email on a mobile device. And in LinkedIn's continued quest for world domination (the stated goal is to have no fewer than 3 billion professional profiles, one for every single member of the global workforce), the company wants to put its network where your eyeballs are: in the email app you're already using.