One of my members put a pole on my site, and the results were a little surprising.
How did you find the site?
Google Search ---------- 94 ------ 31.97%
Kev's Youtube Video's----- 140 ------ 47.96%
From a Friend ------------ 13 ------ 4.42%
Other - Explain in post ---- 46 ------ 15.65%
Google: As of today, my site has 79,500 pages indexed with google. My site is ranked between #1 and #6 in certain key word searches for the nitch of my site.
Youtube: I have a youtube partner account, my videos are watermarked and my channel is branded. My 280 videos have been viewed over 1,300,000 times and I have well over 2,000 subscribers. Currently, my youtube channel gets about 20 new subscribers daily.
Traffic: Google refers about 3X the traffic to my site that youtube does. But, the people that come from youtube are about 140% more likely to sign up for an account and post something as compared to the people that find my site from google.
My first website had @850 members in the first year (2006 - 2007). For the first year I did what a lot of people here are doing - building back links, listing in directories, blah, blah and more blah. The second year I took the site down, reopened it with Vbulletin and started posting youtube videos. The new site grew at about 4X the rate of the old site. I directly attribute most of my forums membership directly to the youtube videos I posted.
My traffic and my youtube videos are directly related. If traffic grows stale, or levels out, I can post a series and videos. In the following days, my traffic goes back up.
When I signed up for a youtube partner account, and branded my youtube channel - the very next day my site got a spike of 500+ uniques. Branding gives the channel owner the option to put a banner next to every video. So that means I have 280 banners on one of the top websites in the world - beat that.

Also, branding gives the channel owner the option to put a banner at the top of their profile page.
Conclusion: If you do not have a youtube account, and if you are not posting videos to promote your site, you are missing out on a large audience.
Something to keep in mind - if I can do it, you can do it. So if you site is lagging behind in memberships and traffic, consider starting a youtube channel.