When we sponsored Mint Flag for the English learning game app "Magna and the Mysterious Girl," we told them not to do any promotions. Large scale promotion with a lot of money invested in TV commercials and Internet ads is generally considered a good product marketing strategy.
This kind of promotion does indeed get the product out there, but unless you've made something really good, the promotion will lose steam after a while.
So no matter how much you promote, the effects won't last for long until the world has accepted that this is an amazing product and it is generating a lot of buzz on the Internet.
I don't mean to throw out promotions entirely. I'm just saying that there is a right time to do it, and you can't do it too early. I don't think it's too late to do it after the product has proven itself. Actually, even though Magna's plot is complete, it will take another five or ten years before we can call it amazing. It's a grand plot, so I see no need to rush through it.