I don't believe Twitter and Facebook are painkillers. Just ask yourself, Is "acetaminophen" really the drug you feel like you're on when you're using Twitter? Or does "methamphetamine" sound more appropriate? There's in fact been research to show that your brain reacts to products like Twitter in much the same way as it would to stimulants like meth.
So what's my point? If you focus only on painkillers, you'll likely miss out on a completely different, and potentially much larger, set of opportunities: those that target pleasure. Pornography, sports and coffee are, for example, three insanely lucrative industries, and each of them sells the promise of pleasure.
Good point, though I'll add that if you are going to focus on pleasure, make sure you're not building an MDMA or weed app (i.e. a non-addictive drug). You have to aim for crack, get your users hooked and coming back for more all the time.