The last few years have seen social networking sites like MySpace, Facebook and Twitter balloon in popularity, but the next big thing in social networking might be the widespread adoption of mobile versions of such services that are designed specifically for use while on the go via smartphones and other handheld devices.

In fact, market research firm ABI Research predicts that some 140 million people will be using mobile social networks by 2013, and they'll generate more than US$410 million in associated revenue. Those numbers may even be conservative. According to In-Stat, another market research company, as many as 975 million people could be using mobile social networks in 2012.

Michael Wolf, ABI Research director of research, says mobile social network subscription numbers will grow at a slow but steady rate during the next three or four years and will then begin to spike when such emerging markets as Brazil, Russia, India and China expand their use of such sites and services.

Wolf also suggests that social network providers will begin charging for use of their services, many of which are currently free. A likely fee would be $1.99 or $2.99 a month for access to a related mobile application, he says.

There are currently five types of mobile social networks, according to In-Stat:

• SMS Messaging Networks like Wadja
• Friend/Community Networks such as Facebook
• Personal Content Networks (Photos and Blogging) like Flickr
• Location-Based Social Networks such as Brightkite
• Dating Networks like Passions Network

A few examples of lesser-known mobile-specific social networking sites are ZYB, MocoSpace and Mig 33.

Mobile versions of the leading social networking services including Facebook, MySpace and Twitter are already available, and there are even a number of free, device specific mobile social networking applications, such as Facebook for BlackBerry, MySpace Mobile and TwitterBerry.

Currently, the largest markets for Mobile Web 2.0 are the Far East and China, Western Europe and North America, Juniper Research says. However, significant growth is expected over the coming years in developing regions like India, South America, Africa and the Middle East, Eastern Europe and the remaining parts of Asia, with these regions collectively eclipsing the current leaders by 2014, according to Juniper.