Some key strategies and tactics I've used on several social-networking heavy campaigns, which yielded the best results were:
- User-End Convenience
- User-Generated Content
- Custom / Personalized Viral Content
- Community Showcase
User-End Convenience could be anything that makes it easier for the community to 'spread the word' or 'forward' on your viral marketing materials. Whether it be a video, a graphic, or URL.
The easier you make it for them to forward to friends the faster and more widespread your viral reach will be. For example, allow copy and paste of the HTML code for a banner or embed code for a video so they can easily and without knowledge of code place it on profiles, in message boards, on blogs, or other media outlets of their choosing.
User-Generated Content or any other form of getting the community or recipients of your viral marketing involved in the campaign itself; whether in the means of creating custom banners, icons, wallpapers, or other graphics for use in the campaign, which other members can vote on and share with each their friends to show what they have created.
Custom / Personalized Content; Much in the vein of the above stated suggestion, this is typically more personal and something they will place solely on they're profile or blog and not someone else's. For example a custom template that expresses their unique tastes or appearance.
For example, I was doing a campaign for a new music oriented site and released several pre-made graphics that had a photo of a member of the site, with a fancy vector-art [huge at the time] frame around it that had their name in one graphic and in the opposite corner said 'I'm In; Why Aren't You?'.
Each time a new member joined, their default photo used to make another graphic along with that members first name and a new bulletin sent out stating 'Amy's in, Why Aren't You' with the new custom Amy graphic included.
Before long, members were joining up and replying to the ads simply to get a custom graphic with their photo that they could place on their profiles and many used as their main picture.
This leads into Community Showcase. Make sure to remind others that 'everyone is doing it' sort of mentality. The Example above with the repeated bulletins showcasing the latest member really gave that feeling that a lot of people are joining up and that theres a reason for it. Hence; the buzz.
Showcase the success of your community for your campaign every chance you get, people always want to be apart of what everyone else is apart of.
Those are some basic ones that works well for me on various projects. Hopefully they'll help you out or at least get the creative juices flowing.