unfan

The act of becoming a fan of an entity on Facebook in order to publicly criticise said entity (verb), or a person who does this (noun), as exemplified by the mass criticism on Nestlé's Facebook page over the company's use of palm oil and subsequent censoring of its online presences.

Has parallel behaviours in other social media (though the irony doesn't translate as well in more language-neutral cases like Twitter's 'follow' function).