Complete tangent, since this has nothing to do with what Facebook are looking into here, but:
If you control both ends of the connection (like Facebook does in WhatsApp), you can match on advertising keywords securely. By doing it client-side, after the text is decrypted.
As I understand it, this is essentially what Apple's iAds framework does: all possible advertising content that could be run in an iAd view gets pre-cached on the client; the iAds framework—running inside the client's address space—then chooses what to show the user, selecting ads based on demographic/interest information it has available locally on the device, never leaked outside. The iAd framework then collects just the impression metrics — how many times each ad was shown to a given user — and then every-so-often batches those metrics out to Apple's servers. Apple then reports re-groups and anonymizes these metrics, and reports them to the advertiser / charges the advertiser for them.
Of course, this means that advertisers need to be able to trust Apple's impression reporting; and to do that, they need to trust that Apple has set up iOS devices so that iAd selection + reporting is basically inviolable, such that nobody on the client end is generating false impressions. (Or, at least, that devices generating such impressions can be detected as being jailbroken through some mechanism, and so their iAd impression reports culled from the corpus.)