While it's possible that the hack will turn out to be a substantive assault on CENTCOM's actual computer systems, thus far the hackers only appear to have gained access to the social media account passwords. The accounts are posting copies of what they claim to be classified military secrets, but in fact many of the documents are publicly available — they seem to have come from public sources like MIT's Lincoln Library and non-governmental think tanks.
If that's right, then this classic comic from XKCD sums up the reasons why we shouldn't freak out about this:
In other words, while it would certainly be better if hackers had not gained access to CENTCOM's social media accounts, we shouldn't worry too much: that's a long, long way from them gaining access to actual sensitive information.
(It's worth noting that the XKCD comic was discussing a DDOS attack, which forces websites offline by flooding them with traffic, while the CENTCOM hack instead took over social media accounts. But the effect is similarly superficial.)