Marketing Your Personal Projects [video]
discovermeteor.com
discovermeteor.com
Or you can approach it the other way around. If you have 1000 emails and you expect to convert 5% of them for a $20 product, you know you'll earn $1000 from your current audience, and can plan from there whether or not that's enough to justify working on a project.
Those are all random numbers obviously, but you get the idea.
My point is that thinking of your audience as "everybody" isn't very helpful, so it can be a good idea to artificially restrict it to a particular segment even if (like Tumblr, Twitter, etc. did) you intend to eventually conquer the whole planet :)
So if I was building Tumblr today, I would first try building an audience in communities that I'm already familiar with, such as designers or the HN crowd.