Benjamin's points are good ones along with other comments above. Some thoughts:
Don't believe your own hype
Most of us "eat our own dog food" as the cliche goes a bit too much and think because we believe our product is wonderful and incredible and magical that everyone else (media, businesses, consumers) will feel the same way. Often doesn't happen that way. Having proofpoints (especially happy customers, case studies, success stories) is a key element of getting people to think it's wonderful, incredible and the rest. Plus often it takes time to get the product "closer to right" than when you launch. Real customers, real feedback, real product enhancements. That will ultimately help a lot.
The product big bang
A huge part of the answer is your product and the potential it has to create the big bang with no marketing. That is true for a lot of the big web properties. Some seemingly explode like Twitter but it was because it delivered a service that nobody else did. It wasn't because they spent a bunch of money on marketing, the product caused the 'big bang'. Likewise with YouTube. Even though there was already online video there was something that they offered that the others didn't which caused it to explode. It hit a spot of need, even in spite of competitors out there, and exploded quickly.
Cost of customer acquisition
Ultimately that's what it's all about. How much does it cost to acquire a customer via SEM, ad banners, partnerships, affiliate marketing, PR, however you market the product? Then how much can each of those scale? So SEM may be my lowest cost per customer acquisition (hah) but it may not be able to get the scale in numbers you want.
And to the point of fast vs. slow, typically with new ventures it's going to take time until you really know the acquisition costs. And those should improve over time as you tune your marketing messages to improve the traffic they drive, strengthen your credibility which helps effectiveness of your efforts, tune your site to improve conversions, tune your tactics to improve your overall acquisition costs. At the same time you have to be opportunistic.
All the best,