Agreed. Trademark law seems to be aimed at preventing companies from pretending to be some other company. This clearly doesn't apply to generic names, like "Sofa Vacuum" or "Photocopier Repair". But Facebook is something that's become de facto ubiquitous, a service almost universally used in the US - more ubiquitous than many generic things.
Surely, then, in the case of companies that provide services that relate to Facebook, there's justification to make direct reference to FB in their names? The major consideration is whether there's much likelihood of these companies being mistaken for the original company - but a name like "Purity for Facebook" (or even "FB Purity") seems like it wouldn't be confused as being from Facebook itself.
Companies are obliged to defend their trademarks, but I hope sanity prevails in the courts, and people get to call their applications "Windows RegistryMaster" or "Mac Backpack" or anything else that isn't confusing or misleading. I'm probably underestimating how messy it'll be to sort out justified (descriptive) usages from the ones that are just piggybacking, though.
He could describe the product as something that purifies Facebook, but that's much different from actually naming the product "FB Purify."
Facebook would argue that the usage is piggy-backing on the goodwill in the name (as with the coffee shop example) but in this case, I think there is a very good reason why 'FB' is being utilised.
Personally speaking, I would argue this is why they are using other means to restrict the growth of the extension rather than using a more traditional cease and desist letter more geared to trade mark rights (at least as far as I'm aware).
[0] http://en.wikipedia.org/wiki/Nominative_use