The DNS was not exactly hijacked, there were issues inside of LinkedIn's top level DNS provider whom were delegating www.linkedin.com authorization to unauthorized nameservers, namely NS[SOMETHING].ztomy.com. The ztomy DNS replaces its delegated domains to point to a domain parking page if there is no record exiting. These changes were then propagated to other nameservers and thus to the end user. End result, dns doesn't point where you think it does.
Au contraire; having the delegation going somewhere unwanted is practically the definition of a DNS hijack. The question is - how did that happen? A malicious third party? a blundering sysadmin? or a bug in some provisioning code?
It does sound like LinkedIn's NOC are playing the blame game already. Well, I guess they've gotta get all those spamming recruiters & sales reps back online.