This sounds quite ludicrous, if it was a request I would understand, but telling you to do this isn't the right way of going about it.
The underlying question here is - are you happy doing this? If your not happy doing this may be you should speak up. I personally wouldn't do it because the way I see it is, you're recommended or vouching for a product which you might not want to, and on top of that, your not even getting paid.
If your employer is really concerned about those numbers (When I'm sure he could be focusing on something else which adds more value), there's plenty of services that sell FB likes out there. Recommend that to him, or even better, resell it to him.