iMedia has a great article about how to track and measure viral campaigns. This is a virtual template for reporting and measuring campaigns.
The article discuses how, while unique to each campaign, some examples of pre-campaign metrics are:
Number of pass-alongs or forwards. Most pass-alongs will happen one of two ways: either via a viral medium or through the email/mobile medium. A viral medium is something designed specifically for effective pass-alongs. For example, you could put a link that says "forward this message to a friend" in your viral email marketing campaign. If the recipient uses this link, you'll be able to track the number exactly. However, if the recipient uses an email or mobile pass-along, it will not be captured or measured. If they hit the forward button through their Outlook or Yahoo! email, it's nearly impossible to track. That's why it's important to include a clear, easy-to-use viral medium to avoid this untrackable option.
Number of embeds. How many people embedded it on their sites? Viral messages embedded in social networks can be powerful -- if the recipient embeds your link onto a well trafficked social networking site, your exposure increases. However, the flipside is also true. The embedded material can become part of a spam blog and the numbers can be skewed. One way to discount distorted numbers is to allocate higher weights to early clicks while also counting unique clicks in that time frame.
Number of comments in the comment field. Some social networking sites let you add comments to what you are viewing. If you include this option, it's important to keep track of the comments added.
Number of downloads. This is a pretty straightforward metric. If there are downloadable elements in your viral marketing campaign, track the number of downloads by each recipient.
Post-campaign analysis
The second phase of measuring your viral marketing campaign involves conducting a full post-campaign analysis. There are a few straightforward numbers that you might already have on your list to collect. Common data sets include:Total number of views
Total unique viewers
Average number of views per visit
Number of conversions
Number of clickthroughs and the clickthrough rate
Number of referrer URLs and examples of referrers, which typically tell you where your users are coming from
Number of email addresses collected
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