No matter the challenges, Facebook still has one thing going for it that will keep it solid for years to come: switching costs. Rebuilding a social network, from the ground up, is hard and annoying work (sorry, G+, but it's true).<p>Back in the day, switching from Yahoo to Google was as simple as entering a new URL into one's browser. Switching from AOL to a different ISP was slightly more difficult, but not much more difficult than cancelling one subscription and signing up for another.<p>But if you look at the landscape of interest-based or activity-based social networking utilities today -- Pinterest, Tumblr, and so forth -- they really don't look like Facebook killers. In fact, many of them have integrated Facebook credentialing. In aggregate, they may take user eyeballs off of the Facebook homepage -- but they aren't taking user accounts away from Facebook.<p>Facebook seems to realize that the key to its longevity will be as a de facto social and identity credential, and not necessarily as a destination website (although that's nice, too). People may not want to spend as much time on Facebook.com as they once did, but it will take a <i>lot</i> to get them to abandon the Facebook ecosystem altogether.<p>Accordingly, I submit that the real challenge Facebook faces isn't monetizing display ad space on Facebook.com; it's monetizing the rest of the internet as credentialed by Facebook. (To your point, the "off-Facebook" ad platform). This challenge isn't insurmountable, either. In fact, it doesn't strain the imagination to think of an AdSense-competitor product that could serve up contextually relevant advertising to FB-credentialed users as they surfed the rest of the web.