While I'm not a marketer, targeting anyone and everyone isn't the way to go, imo. Despite all the greatness we think that we built, most people simply won't care. And spam is the evil that doesn't think it's evil because it thinks it's doing everyone else a service.

"And I'm not saying that television is vulgar and dumb because the people who compose the Audience are vulgar and dumb. Television is the way it is simply because people tend to be extremely similar in their vulgar and prurient and dumb interests and wildly different in their refined and aesthetic and noble interests." X David Foster Wallace

While the guy is a novelist, and not in television, he's done work in observing the effects of television on fiction. But, I think it makes sense.

Given that, unless your service is pretty basic, like water and sun of the internet, or pretty vulgar, like porn (also appeal to basic affinity towards sex), it will probably only first appeal to those that are interested in that particular niche.

This is why finding those people that are fans or first movers in that particular field/industry is important, and especially if they're obsessed enough to keep a blog on it, due to a need they have to tell other people. For a new market however, you'll need to find a cross-section that cuts across other groups--this, I have no idea how to do.

Anyway, that's why it's usually considered a good idea to pick problems that are your problems to work on for a startup, because in addition to scratching your own itch, you'll know where to find other complaining fools (besides yourself) that have the same problem. If you can do it well, they'll tell others.