However, the return on investment in these campaigns is very low, and marketers should be prepared to engage in a personal relationship with users by providing something of value.
Promotions are good in this context, according to Forrester, but information or brand elements that users can pass on to their friends are even better.
"It is clear that successful social networking site campaigns do not follow traditional marketing rules," said Charlene Li, a principal analyst at Forrester and co-author of the report.
"Social networking sites cannot be treated as channels because their members are not passive web pages."
The report suggests that marketers should mimic how music acts promote themselves on sites like MySpace by engaging their fans with frequent backstage gossip and answering their questions.
"During the past 10 years, the evolution of the internet has dramatically changed how organisations interact with customers," said Gurval Caer, president and chief executive at marketing agency Blast Radius.
"Companies are recognising that traditional marketing approaches like advertising are less effective today, and marketers are struggling to deliver value.
"People no longer want 'interruptive' brand communications; they want interactions with their peers and true value from companies through Facebook applications or communities for sharing ideas and experiences."
Caer added that marketing needs to "turn itself on its head" with a much greater focus on building relationships that will make people's lives "easier, better and richer".
The report concluded that companies that want to advertise on social networks should embrace the interactive aspect of the sites in order to gain the full benefit of these campaigns.
- Forrester Report: Marketing on Social Networking Sites