Aktimel,
The best advice I can offer with regards to growth curve is to try to analyze which existing web sites are "similar" to yours, with regards to interaction model (edited content site, user generated content, one to one, one to many, etc), domain, "viralness", etc.
Once you've figured that out, pick from the similar services one that is considered a success and one that is considered as less-of-a-success (e.g. failure). Then, try to find out the traffic stats data points for these two services in their first year, and create your projections based on these.