One can trade data navigability and a performance hit for opacity of content.
Encrypted rows of data are meaningless to an "admin" that can query to its heart's content but will never be able to decrypt the result set. On the other hand, the layers on top (such as the web-tier that emits the plaintext) may have the keys to decrypt, but lack the privs to run around in the database; from that level, they must pass along the user's credentials to obtain user specific content.
Since people don't search by content on Twitter (afaik) and only 'meta-data' indexes are used (such as hash-tags, follower, following, date) this is entirely doable for something like Twitter.
There is also 'Homomorphic Encryption', but I'm not sure the tech there has reached acceptable performance levels.
