Based on research, the predominant demographics are listed for several of the social media sites.

Demographics:
Female, teens and up to age 50+, Caucasian with more African American visitors than usual, children 0 to 17 in household, more
affluent $60 - $100k+, high index of college graduates.
Facebook is a top 10 site that reaches over 91 million US visitors monthly. Facebook allows an advertiser to place a text ad with a graphic and link to the client’s website. The client can target their audience by choosing certain demographics (age, gender, interests, communities, etc.)
Text ads from the Google AdWords Content Network can also be displayed on Facebook.

Demographics:
Female, young to middle-aged adults (18-49), Caucasian with more African American visitors than usual, children 0 – 2 yrs.
in household, less affluent (less than $60k), high index of college graduates.

Demographics:
Male, teens to young adult, Caucasian with more Hispanic visitors than average, children 0 -17 in household, less affluent
($30k or less), people with no college education, reaches over 72 million US people monthly. You can advertise on You Tube with “text” ads, promoted by
Google AdWords Content Network advertising or videos promoted through a You Tube account.
Client must provide all video for uploading. Promotion Text is created and keywords are chosen to trigger your ads. Unlike traditional pay-per-click advertising, the keywords should be broader in nature to attract a large group of viewers. The client is charged when someone clicks on their video. The costs per click can be as low as a few cents and a daily advertising budget can be set.
You Tube has four different targeting options: demographic, Internet based, content and buzz targeting.

Demographics:
Male, middle-aged, Caucasian, with more Asian visitors than average, no children in the household, more affluent ($100k+),
college graduates and post-graduates. Linkedin.com connects marketers to over 40 million affluent, ambitious, and influential professionals.
It is a powerful business network based on relationships and trust. It includes members from all of the Fortune 500 companies, as well as a wide range of
technology, financial services, media, consumer packaged goods, entertainment, fashion industries, and more.
LinkedIn is also backed by world-class companies. Ads contain a link to the advertiser’s website and link to the member or company profile. The link to the company profile is very beneficial in providing “credibility”. All ads must be professional and not contain any “spam-like” content.
Linkedin allows you to choose “the market”, based on demographics such as occupation, gender, and employment status.

Demographic:
Female, teen, Caucasian with more Hispanic visitors than average, children 0 – 17 in household, less affluent (less than 60k),
people with no college education, reaches over 63 million monthly in US.
Advertising on My Space is similar to search engine pay-per-click advertising using a daily budget. However, instead of keywords, you’re ads are shown based on multiple targeting options such as location, age, gender and education. There are also over 1,100 interest categories to help target your audience such as sports, music, politics, etc.
The ads can be top banner, side banner or block and consist of an image, along with corresponding text. The ad will lead to the destination URL of your choice. Text ads from the Google AdWords Content Network can also be displayed on My Space.
Technorati is an Internet search engine for searching blogs. By June 2008, Technorati was indexing 112.8 million blogs and over 250 million pieces of tagged social media. Technorati uses and contributes to open source software. Technorati has an active software developer community, many of them from open-source culture.
The site won the SXSW 2006 awards for Best Technical Achievement and also Best of Show. It was also nominated for a 2006 Webby Award for Best Practices, but lost to Flickr and Google Maps.
Technorati rates each blog's "authority", the number of unique blogs linking to the blog over the previous six months. In 2009 Technorati decided to stop indexing blogs and sites in languages other than English in order to focus only on the English Blogosphere. As a result, thousands of sites in various languages are no longer rated by the Technorati service.
Overall, Social Media Advertising offers a new option for Internet Advertisers. The power of “social networking” can be a huge benefit to a company. If the time and markets are right for you, join in and let the internet work for you!