Focus is important in social media. Without focus it can be like boiling the ocean. So when marketers new to social media begin they wonder which social networks they should focus on. Twitter? LinkedIn? Do they need a Facebook Fan Page? How about MySpace? A YouTube Channel? Flickr? The list goes on.

Wikipedia maintains a list of social networking websites with a column for registered users. It’s shocking how many social networks there are with over 10 million registered users that I haven’t heard of before. Habbo, a social network for teens has over 160 million registered users, over 8 million unique visitors per month, and 75,000+ new registrations per day!

Marketers should look at two things – Audience reach and search engine optimization (SEO). Many times the two are exclusive. For some clients work on getting lots of content on YouTube and a high volume of impressions when it’s not the best site for their audience. The fact is, the YouTube pages get highly ranked in search engines.

An April 2010 study, Social Media Marketing Industry Report by Michael Stelzner of 1900 marketing professionals showed that by far Twitter, Facebook, LinkedIn and blogs were the top four social media tools used by marketers.

My advice is to target the top 4-5 sites and be sure to include social bookmarking sites separate. You should plan to bookmark new content on a few social bookmarking sites like digg, Stumble Upon, Yahoo! Buzz. Do your research; you may find 1 or 2 sites you haven’t heard of that service your audience. If your audience is teen then discovering Habbo could be a big opportunity.

The CMO’s Guide To The Social Media Landscape from CMO.com, prepared by 97th Floor, an SEO and social media firm, is an interesting take on the top 10 choices.