“What’s the No. 1 activity on mobile? It’s email, and the growth of mobile email is simply staggering,” explained Rapportive founder Rahul Vohra at LinkedIn’s Mobile Day event, held in San Francisco. Four years ago, less than 4% of emails were read on mobile. Currently, that percentage is over 50%. Vohra argues the ability to integrate a LinkedIn user’s profile information directly into email.

Here’s how it works: iOS users with Gmail, Yahoo (YHOO), Aol (AOL), Google (GOOG) Apps, and Apple (AAPL) iCloud e-mail accounts can download LinkedIn Intro, and a pane appears in their current Apple Mail app. If an email recipient is also on LinkedIn, their information pops up within the app, including photo, current position, previous experience, and education among other data.

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For some mobile users, the benefits are obvious. People who are networking and/or seeking a new job can dash off better-informed emails more quickly. “With that smaller screen size, the expectation is that users are going to find content much faster,” LinkedIn CEO Jeff Weiner put simply.

Also at the event, LinkedIn executives disclosed more data about mobile usage. Two-and-a-half years ago, just 8% of members visited the network via mobile; now 38% of LinkedIn’s 238 million users do so. Indeed, Weiner said he expects more than half of users to access LinkedIn in some regions next year.