What is your target audience? Is this a product / service for local, national or international?
What is your advertising budget?
If your retail, like us, we advertised heavily in local and surronding area, local information websites, forums, free papers and for our online store we have advertised in local free papers for other areas - and on the odd site but only 1 as we dont know the popularity of those sites.
If you want to advertise on one particular site ask the site admin for traffic stats to see how many hits (unique and returning) they get, if they wont give them then dont bother as most will be happy to give it to get advertising revenue.