Google’s (NASDAQ:GOOG) YouTube has seen tremendous growth over the last few years. Based on Comscore data, we estimate that the average number of YouTube unique visitors globally increased from around 63 million per month in 2006 to around 375 million per month in 2009. We believe YouTube unique visitors will continue to increase at a slower pace and reach more than 700 million per month by the end of the Trefis forecast period.

We believe YouTube will continue to grow for the following reasons:

1. Fast Growth of Social Networking Media

Social networks like Facebook, MySpace and Twitter have grown rapidly over the last few years and are amongst the most visited websites globally. YouTube will benefit from the growth of social networks that allow users to embed YouTube videos or share links to videos.

2. Ease of Video Uploads and Video Search on YouTube

As more digital content is generated by each person (mobile phone video cameras, flip cams), YouTube’s ease of video upload will lead to further proliferation of YouTube content which will in turn draw in more viewers. YouTube is working to make video search easier to use and video search results more relevant which will make the site even more attractive.

We estimate that YouTube constitutes less than 3% of the $704 Trefis price estimate for Google’s stock. You can modify our forecast for number of YouTube users above to see how Google’s stock would be impacted if it were to grow faster than we forecast.

For additional analysis and forecasts, here is our complete model for Google’s stock.