More to explore:
Facebook fans loyality
59% of consumers have ‘liked’ a Facebook brand page in the past 6 months
Impact of a Facebook Like
social media to marketers: get over yourself
Impact that 'Liking' a Brand on Facebook Has on Purchase Intent of US Facebook Users, Sep 2011
Does 'Liking' a Brand Drive User Loyalty
I don't think it impacts loyalty as much as brand awareness that may help with acquisition. Loyalty in my mind comes after the brand introduction or "like".