First, they suck. I buy a camera, like it, don't return it, yet am then bombarded by ads from the vendor - .
I typically use blocking software on most sites, but not all. What I find is the ads are always too late to be useful.
As an example, I was looking at AutoTrader for a used car. Found one I liked, went to dealer, and purchased it. Now, weeks later I still get ads for vehicles. No problem; maybe they assume people search longer for a car. I'll buy that.
Purchased flowers for my fiance and the next day I'm getting ads for flowers. Yes, I love her but I'm not buying flowers everyday.
Basically what I'm trying to say is that the ads lag behind what I'm in the market for. They aren't predictive (maybe Google does better when scanning your email) and thus don't add much value if you're already done with the purchase. Facebook seems a bit better because they link them to topics people are discussing in posts. I like to post about cool guitar gear I find so an ad for a discount at Guitar Center might be useful. However, that is rare too.
BTW, don't like ads anymore than anyone else.