Yes.

Ah, that's probably too short an answer, so let's expand upon the idea here, because it's a very important one: if you can find a venue online where you can gain some traffic or visibility, even if most of it's within that venue exclusively, why wouldn't you want to have a presence? I have a MySpace profile, for example (Dave's MySpace Profile) and while I certainly don't consider myself a particularly passionate MySpace user, it's definitely the case that some of my customers, my readers, are quite involved and they like being able to connect with me on their virtual turf.

Same with YouTube and its sister, Google Video: If you have some video content of interest that you can upload, something that perhaps promotes your site, product or service in a witty or amusing way, I strongly counsel you to do so, and then link to it from your blog, your Web site, and even email your favorite dozen customers to let them know about it!

As you cast a wider net, however, some of the sites that are popular might not prove to be good venues for your promotional efforts, just by the nature of their existing communities. For example, most affiliate marketing folk aren't going to find customers through the college-focused Facebook social networking site. If you're selling next-morning hangover cures, however, maybe it would be worth exploring that possibility.

Bookmarking and "resource identification sites" like Del.icio.us, Digg and Reddit are worth leveraging as you can, but it's difficult to gain any traction unless you have a veritable army of customers who can "vote" your information to the top of the pile on those sites, and generally that kind of fraud can be easily detected and blocked anyway, so it probably isn't a good idea. In fact, forget I said that at all!

Instead, spend some time thinking about and then adding valuable, helfpul, and informative comments on popular blog sites that demonstrate your expertise in your market segment. Don't have explicit links in the 'body' of your comment, but do point to your site in the URL field that's an optional value to specify when commenting. You won't get a 100% click-thru rate, but even if it's 5% that's still 5% more than you'd have without that contributing to the discussion.

Hope that helps you identify fruitful avenues for further research as you try to generate more customers (not traffic! customers!) to your site.