This list contains 30 groups, ranked according to the size of their current membership.

1. eMarketing Association Network (202,892) - Largest and most active Marketing Group with 202,000+ members. Managed by the eMarketing Association.

2. Innovative Marketing, PR, Sales, Word-of-Mouth & Buzz Innovators (79,419) - Innovate with innovative marketing, public relations, promotions sales & selling professionals skilled at using creative marketing methods.

3. Media Professionals Worldwide (67,149) - Media & Entertainment group for media professionals from USA (America), Europe, Asia (China & India), Australia etc.

4. ThoseinMedia (60,435) - A group for Social, Online, & Broadcast Media, Ad Sales, Marketing, Buying/Planning, Interactive, Gaming, Print, TV, Radio, Mobile, Internet, Animation, Newspaper, PR, SEM, Paid & Organic Search, SEO, CPM, CPC, CPA, Analytics, Publishing, Ad Agency.

5. Social Media Marketing (53,387) – This is the LARGEST and most active social media group on LinkedIn.com. It is intended for interactive advertising, marketing and other professionals who are actively engaged in social media and community-oriented websites.

6. Online Advertising Professionals (49,987) - Online Advertising Professionals forum for discussing ideas, exchanging information.

7. Inbound Marketers – For Marketing Professionals (40,777) - Online group for marketing professionals. A community for those looking to reach their best customers online through techniques like inbound marketing, search engine optimization (SEO) and social media.

8. eOffice (39,127) – Join if you are interested in the latest trends in design, technology, workplace environments, worklife balance. eOffice offers state-of-the-art flexible office space, meeting, conference rooms, virtual offices in London and worldwide, via its eNetwork.

9. Digital Marketing (30,311) - A group for people who want to engage in debate and exchange information on best practice digital marketing.  It’s ideally designed for Digital marketing, internet and e-commerce professionals.

10. Marketing & Communication Network (28,767) – A great group for marketing and sales professionals.

11. Advertising Professional (26,356) – For Professionals who work in Advertising Market.

12. Facebook.com (22,503) – A LinkedIn group for Facebook fans.

13. Google Group (22,305) - This group is for people who use Google as their primary search engine.

14. E-Commerce Network (19,625) - For people who are interested in E-Commence and its future trend.

15. Public Relations and Communications Professionals (19,585) – A group of Public Relations and Communications Professionals dedicated to increasing networking and business opportunities for its members.

16. Women 2.0 (18,690) - Networking women in entrepreneurship, business, and technology.

17. Social Media Today (17,512) – A social media group for tech savvy people and Web 2.0 lovers.

18. Marketing Communication (17,399) - A group designed for people interested in marketing and communications.

19. Creative Design Pros (17,383) – A design community that boasts a cross section of creative pros in fashion, industrial, advertising, web design, etc.

20. Web 2.0 (16,062) - Web 2.0 group is a network of professionals involved in the 2nd generation of web-based communities & hosted services, such as social-networking sites, wikis and folksonomies, which aim to facilitate collaboration and sharing between users.

21. Animation, Media & Entertainment (15,227) - Enable networking & information sharing opportunities for professionals in the animation, gaming, cinema, television and all other formats of media & entertainment businesses.

22. Advertising Professionals (14,562) - This is a group for advertising professionals of the world.

23. Future Social Media (13,522) - The essential online meeting place for people captivated by the transformational power of social media.

24. Technology Marketing Community (12,850) - This is a group for marketing professionals working in high-tech industries to build a network that connects people, opportunities, and ideas.

25. Twitter innovators Innovation & Creativity Network of Tweeters (12,132) - A LinkedIn group for Twitterers.  Many members are experts on innovation or passionate about using creativity to Twitter. Exchange questions, ideas & advice.

26. Sports Marketing 2.0 (10,457) – This group is an international, digital think-tank for sports marketers in the Web 2.0 world.

27. Search Engine Strategies Conference & Expo (10,090) - Search Engine Strategies, the conference and expo series for search marketers (SEO & SEM), was launched in 1999. With events around the world SES has taught many thousands about paid and organic search, social media strategies and integrated marketing analytics. A good group to join if you are interested in search engine optimization.

28. Search Engine Land (9,798) – A home for Search Engine Land readers on LinkedIn.

29. Search Marketing Salon (7,223) - Search Marketing Salon is a global network of Search Engine Marketers. It’s open to anyone who is obsessed about increasing search engine rankings, driving targeted traffic and maximizing revenue through paid search (PPC), search engine optimization (SEO), or Web 2.0 Marketing (and social SEO).

30. Twitter Power for Business (5,761) - Based on the book Twitter Power written by Joel Comm.  Another great LinkedIn group to join if you like Twitter.

Although nowhere near the top in terms of members, I've gotta give a mention to our newly created Performance Marketing - Channel Intelligence linkedin group. (134) - We provide tips and insights into marketing channel valuation and attribution to help understand the buying path of offline/online as well as articles related to dynamic display, product search, social commerce, and where-to-buy.

Note: All groups were ranked based on the number of group members as of 1/16/2010. The number may have changed over time.

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