This week we're reviewing five trends that have helped define 2011. So far we've covered online privacy, group messaging and HTML5. Today we're looking at a still emerging trend, but one which is (pardon the pun) very much worth watching: second screen apps. By that we mean apps that run on your smartphone or tablet device and complement your television viewing.

Over 2011, second screen apps have continued their push into the living room. Watching TV used to be a passive activity, but now the Web and devices like Android phones and the iPad have made it interactive.

According to a study published in January by Yahoo's advertising division, 86% of people who use the mobile Internet use their mobile device while watching television. Of those using their mobile device, 25% say they are browsing content related to the program they are watching.

Social TV

Despite the increasing use of time-shifting devices like DVRs and TiVO - which allow you to record and watch TV shows at any time - there has been a lot of growth in synchronous social networking around TV shows.

GetGlue is an app that we've tracked closely in this space and in April this year it announced 1 million users. At that time it also had 100 million data points - likes, reviews, check-ins - representing connections between entertainment and people.

With GetGlue, essentially you 'check in' to a TV show like you would check in to a venue on Foursquare. This allows you to talk about the show with other people who are also watching it. This activity is becoming increasingly popular, at least according to GetGlue. April turned out to be a high point for the company, with about 4 million check-ins.

Other companies in this space include IntoNow (acquired in April by Yahoo!), Miso and PHILO.

TV Tweeting

When it comes to real-time social networking, Twitter is where a lot of that activity happens. Including around TV shows. In January, Twitter CEO Dick Costolo discussed what he called "the Glee phenomena" at the Consumer Electronics Show.

"The characters on Glee actually tweet and they tweet during the show. When Glee starts, the moment it airs for the first time on the East Coast, the tweets per second for Glee shoot up," said Costolo. "They stay up there at a super high level at hundreds of [times] what they are before the show comes on until the moment the show ends and then they drop. [...] People feel like they have to watch the show while it's going on because the community is tweeting about the show and the characters are tweeting as the show's happening so [they have to] watch it in real time."

But Wait, There's More...

Recently we profiled a product called WiO, which enables TV watchers to immediately get information about the products and services they see advertised on screen, both in TV commercials and within the shows themselves. Through a mobile app running on customers' phones, marketers can offer a variety of follow-up actions to the TV viewer. These include coupons, reminders, contact info and more. In total, there are 10 follow-up actions offered. The consumer controls which ones, if any, they respond to.

Internet TV Continues to Ramp Up

Finally we should mention the Internet TV efforts of various companies, such as Google TV and set-top boxes like Roku and Boxee. These aren't second screen apps, but all of them will complement those apps and some will even be development platforms for them (e.g. Google TV). Also it's worth noting that these services are steadily eroding the audience of cable television, so in that sense they are becoming increasingly important to second screen apps.

The Internet TV ecosystem is growing. To take one example, in May Roku announced that it had reached 15 million channel downloads from more than one million people on its service. The "big four" channels, as Roku calls them, are Netflix, Hulu Plus, Pandora and Amazon Instant Video. What's more, Roku estimates that 15% to 20% of its users are cutting the cord on cable television.

Let us know in the comments which second screen apps you've started to use and your thoughts on them.