Having a higher-than-ideal concentration of referral channels is not in and of itself a bad thing if you truly understand those channels. For example, a business that derives the majority of its customer referrals from paid search doesn't necessarily have a valid reason to diversify if it has the ROI equation down and understands and manages the associated risks.
The real problem with social media channels like Facebook is that many folks have been treating them (and investing in them) as if they were "owned media" when they never have been and never will be. These people are just figuring out that these channels fall under the categories of "earned media" and "paid media" now that Facebook et. al. are turning the screws. The true tragedy of this is that many of these unsophisticated marketers will have little to nothing to show for their social investments when all is said and done. They've been spending gobs of time and money building up their social profiles and will now have to pay even more in perpetuity to use them going forward.