i'm quite surprized this hasn't been asked yet
so for a given web app and a given set of potential users of this web app the goal would be to reach this set of potential users. how to achieve this?
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De question here is what size the audience is?
If it is big, then you just advertise where most people will see it.
If it is smaller, then you target for the common features of the audience, and find websites, magazines, and the sort of stuff, they get in contact with.
But as allways, the best ad. is a happy customer.
And be prepared that you will spend more on this, than the development of the thing. And by More, i mean three or four fold to begin with.