Be careful where you choose to advertise. Publishers (online or not) will always boast about how many readers they have. However, they may not translate into sales for you.
The obvious reason may be a lack of targeting: if you sell toys, a magazine about gardening is not going to produce results regardless of the size of their readership.
Another reason is what people are doing at the time of seeing your ad. For instance online forums are usually well attended and highly focused, however when people are involved in a discussion they are unlikely to react to your ads.