Since the rise of the internet, marketing has become easier. Since the rise of blogs, marketing has become much easier. You can now throw hundreds of thousands of people at your service in a matter of hours by getting good coverage in the blogosphere (as a best case scenario). Obviously the majority of those people won't become your customer, but your name will be etched in their mind... It could either be a good "etching", or a bad "etching". If it's the latter, you might be screwed.
That begins the WOM (word of mouth) campaign... As long as you can keep your service innovative and useful, you can maintain a level of growth for years to come. Your selling point is that you always have innovative features and you solve a real problem, so people often refer you to others - that's what I define as basic virility. Under this definition is MySpace - they didn't rely on external widgets to draw the user in - they were just so "cool" that friends bugged friends to sign up. Their features keep people coming to the site.
On the other hand, I define widget virility as something that draws the user in externally using widgets (who would have thought?). For example, YouTube's success was probably a direct result of their embedded video player... Delicious' or FeedBurner's success could be attributed to their chicklet widgets.
I guess the hardest part is getting your service in front of someone. Even if your startup solves everyone's problem, it's worthless if nobody knows about it. It just simply isn't a case of "build it, and they will come". Who are "they"? How will you capture their attention?
The way I imagine Wantsy succeeding is by drawing in users who are interested in purchasing an item - i.e. the existing wanted-ad market. As a previous high-revenue seller on eBay, this would clearly be a driving force for other sellers. If I have an item that I'm getting ready to post on eBay, why not do a quick search on Wantsy first to see if there is an immediate demand? If I'm a buyer, why not create a quick listing of the item that I will be seeking? Even if I plan on using eBay, I can still create a wanted ad and get the best of both worlds.
We aren't competing with eBay, Craigslist, Edgeio, or Etsy - we're simply supplementing them.