Well they're not paying for the service directly, but surely the advertisers want something for their money. And inflated material desires with excessive spending can certainly keep the poor poor, in a way that a fixed $n/month expense can't.
That said, advertising which directs your expenditures to more appropriate places is morally fine by me. If I search for "buy a basketball" and some start-up is able to expose me to a better, cheaper basketball than that seems useful. However, if that same query returns ads for Nike shoes, Kobe Bryant jerseys, and the All-NBA-All-The-Time cable channel, then that's bad for society, I think.
The canonical example I use for myself is grocery store coupons. I purposely ignore them. While intuitively it seems I could save money by redirecting my purchases to things that are on sale, I use my meta-intuition to know that more than likely it will convince me to buy some category or brand of good (now or on a future trip) that I wouldn't have otherwise purchased. Consequently, I see coupons as net money-losers.
I see ads in the same way. While it may get me this service for free, I imagine it planting seeds of desire in my mind for various other goods/services, such that I end up paying more over the course of my life than I would have if I just paid for the service in the first place and didn't see the ads.