After Facebook and Myspace Now Google have also included there ads in social networking sites i.e orkut but there is a huge difference in the way of displaying ads inside orkut as comapred to other social networking sites ,Orkut is following the same way what google search engine follows in putting ads in front of you.
You have observed that google search engine display ads only to related content what you search in search engines ,and the same procedure is followed via orkut in putting advertisment .
Orkut is displaying ads from very begining but they have changed the ways of displaying ads, For ex-when you searched for sms in orkut ,orkut will display all sms related ads in search page and in orkut sidebar which is helpful for you in finding yours related content. 
