Great piece on Epicenter explaining, plausibly, how Facebook could grab Google's ad business by using data they gather better than Google does. Further, even though Google does accumulate some of this data, they can't use because they've promised not to.
The same thing happens on the journalism side of things as well. A few years ago CNET probably felt they could not be routed-around, but they were. It happens regularly, and will certainly happen again.
I have a theory that you can avoid being routed around if you don't try to erect the barriers at all. If you welcome all comers, and try to learn from them, including your competitors. In such a world, Google would have tried to do what Facebook and Twitter do as soon as they came around, without trying to use their installed base advantage to compete. I don't think that organizationally they could use their own people to do it, rather they would invest in the newcomers, aggressively.
One tech company that appears to get this is also a generally quiet company that doesn't bluster much, and tries like hell not to take sides in the technology wars. For 10 points, which company is that? Click on the plus next to this paragraph in the blog post for the answer.