Social media moves so fast, it's hard to keep up. Here are the week's top stories in scan-friendly format:

  • Study: Social Networking Most Popular Online Activity
  • Twitter Fields It's 20 Billionth Tweet
  • Grab A London Cab Via Twitter
  • Facebook Launches Questions Features

Study: Social Networking Most Popular Online Activity

In a recent study, Nielsen concluded that social networking is the most popular activity that Internet users engage in. In fact, social networking is twice as popular as the next post time-consuming activity, online games. According to the figures, online users now spend 22.7% of their time engaging in sites such as MySpace, Facebook and Twitter.

On an interesting note, traditional online activities such as email and Instant messaging is down 28% and 15%, respectively. This would make sense as more folks migrate to Facebook for messaging rather than using email. However, one interesting question lingers: if you're instant messaging while on Facebook; would this be classified as IM or "social networking" to Nielsen.

The overarching theme here is that social networking has replaced all other activities to be a dominant stable of time spent online. Marketers and product developers need to take note of the magnitude of these usage patterns.

Twitter Fields Its 20 Billionth Tweet

There's no denying that Twitter is one of the darlings of the social networking space. With user growth going through the roof and activity ramping up, Twitter is a strong player. Earlier this week, a Japanese designer helped Twitter reach a milestone by posting the 20 billionth tweet. Soon after sending it, the author received a huge batch of replies from folks all over the world.

Twenty billion is a undoubtedly a staggering number. As Marshal Kirkpatrick points out, it would be interesting to know how many tweets are actual messages sent out by humans and how many are bots or advertising spam. Google Buzz has advertised the services' amount of messages, but most of these messages are made automatically by Flickr photos, blog posts and other automated messages.

With Twitter being so big now, how will the company turn these usage numbers into revenue and how will the company continue its dramatic growth?  These are the looming questions surrounding the popular micro-messaging service.

Grab A London Cab Via Twitter

Twitter is showing lots of different ways that its users are making the service a much-needed utility. For example, the next time you're seeking a cab in London, simply tweet out a message to @tweetlondoncab. With more than 100 cabs utilizing this method, a co-op of independent cabbies are meeting the needs of tweeting cab customers.

After a tweet is cataloged by a perspective customers, cab drivers tweet back their location and pick up the tweeter if they are nearby. What are the next steps? A few mobile application developers have offered to build mobile apps to help the effort along and the cabs are looking at GPS technology to make the service even quicker for those looking for a cab. By utilizing Twitter as a communication medium, these cabbies are finding business in a decentralized manner that helps them make revenue and helps cab customers find cabs quickly and easily.

Facebook Launches Questions Features

Facebook is now giving you the ability to pose questions to the whole Facebook community. Facebook Questions allows you to ask a question and field answers in the same way that MetaFilter, Yahoo! Answers and Quora allow you to do. 

Facebook Questions lets you do some unique things surrounding questioning, including: photo questions: take a picture and pose a question about it, such as "What famous figure is this?", polling, tagging and topic exploration.

This is clearly a move to provide yet another utility to make users stick around. Questions has a huge advantage over some other startups such as Quora or Mahalo -- like half a billion Facebook users that are already signed up and ready to use such a service.

Would you use Facebook Questions?  Does the fact that this new service is inside Facebook already make you feel more apt to check it out?