I've been thinking a lot lately about much our industry seems to be trying to leverage addiction. It's certainly not just Facebook and Google -- it seems to be happening throughout consumer technology. For example, video games seem carefully optimized to turn us into Skinner mice, and most television apps have adopted the annoying "play the next episode unless the user opts out in X seconds" feature. (Even Plex, although I'm not sure what they're getting out of it.)
Ultimately, I suppose it's up to the consumer to resist the addiction while availing themselves of the value provided by these products, in much the same way that most of us can enjoy and appreciate beer, wine, and spirits without becoming alcoholics. But thinking about these things is making me reconsider what sort of projects I'd like to work on.
EDIT: Obligatory Paul Graham article on the subject: http://www.paulgraham.com/addiction.html