The TABs are technically hackable, but apart from network data, there isn't too much to find. Where network data is concerned, it's a matter of OOC permission-- a private or encrypted transmission can only be hacked if all players involved are okay with it.

Network data is saved, though, so yes-- unlocked posts and unsecured threads can be read by anyone who has the technical know-how! The only exception would be posts and calls made prior to the current TAB system. The communication system used aboard the Moira relied on a different sort of device (MIDs), so any character attempting to dredge up data from back then would be unable to do so with just their TAB.