An infrastructure disruption at a large colocation provider would probably cause just as much inconvenience to users. Attacks against root name servers can (and have) cause huge amounts of disruption.
I understand your point is about people building upon single points of failure, but my point is that there have always been single points of failure within the internet. I don't think conceptualising facebook as a single point of failure is particularly accurate either, given the distributed nature of its implementation. You're more likely to get localised failures of facebook than a complete outage, which would appear to be exactly the same amount of risk as any other situation which I've described.
As for URL shorteners, it seems apparent that there might be a business opportunity in client-side hashing with a server-side implementation for those without client support. That way you remove the point of failure, which I agree is a good thing, even if we disagree about how important that point of failure is.
A DNS or ISP disruption can usually be more localized. I would argue that it's much easier to disrupt a Twitter or a Facebook than it would to take down multiple DNS servers or multiple ISPs. It would take less resrouces and time to create a global disruption by taking down Twitter than it would to take down an ISP.