I haven't done any promotion for
http://asoftmurmur.com besides posting to reddit and HN, and it now has 400-500 regular daily users. It depends entirely on what type of product or service you're offering.
Another key thing is to understand the community. There is radical cultural diversity between subreddits which manifests in very different reactions to self-promotion. It's essential to engage appropriately and respectfully.
Something I've used for other projects is searching for coverage of competitors in the same space, then pitching to people who have already featured them. Again, very important to hand-tailor each pitch and offer value to the person you're contacting.