Why do we use social media? To put it simply, it’s because we know someone.

According to Nielsen’s 2012 Social Media Report, nearly two-thirds of people who join social media say that they connect with people they know in real life. Another half of users connect with mutual friends. And 42 percent want to keep up on what’s happening. Just 6 percent accept all user requests.

How does that little bit of knowledge fit into your member needs? Good question—and one you should answer soon, because this market is growing fast. More key points from the study:

For mobile users

Apps vs. mobile web: One relevant data point: People spend more time using apps (129.4 billion total minutes) than they do mobile websites (28.1 billion total minutes). Both still pale in comparison to PCs (362.7 billion total minutes), but minutes on mobile apps more than doubled between 2011 and 2012.

Where people use mobile: Slightly more than half of people ages 25 to 34 use social media in the office. One surprising fact reported by the study: Nearly a third of people ages 18 to 24 look at their phone in the, um, bathroom.

For the socially active

First off, check out our graphic of the biggest social media growth points:

So, wow, Pinterest is growing fast—but don’t count out Google+ and Tumblr! Other interesting details:

Pinterest’s major sprouting: It has a huge audience that sprung out of nowhere, and it’s the social media story of the year. The demographic trends that Pinterest are showing are intriguing, by the way: Most of its users are female, and the network does well among users in the 25-49 age group. On top of that, though, the service is hugely popular among Hispanic mobile users, who make up more than 20 percent of both its mobile app and mobile web audience.

Mobile’s social media growth: Nearly a third of all social media activity takes place on apps, with apps (40.8 billion total minutes) far outpacing mobile websites (5.7 billion total minutes). Demographically, female users, Hispanics and people between the ages of 25 and 34 are far more likely to use mobile apps, with mobile social media usage dropping heavily for people over the age of 45.

The top dog slows down: Facebook is still the top social network based on unique users and mobile usage, but it’s growing at a slower clip than most other social networks, with a 4 percent decline in unique PC users and just a 9 percent increase in mobile web usage. But, with 152 million unique visitors among desktop users and 78 million mobile app users, does it really need to grow so quickly anymore?

Where are you finding your best luck on mobile and social? Let us know in the comments.

(iStockphoto/Thinkstock)