The cost is that WotC can no longer control or even direct discussion of their IP and have to instead rely on the whims and foibles of random hosts on the internet.
I agree with you. There's a big difference between controlling your own content, and it being subject to the whims of Mark Zuckerberg or whoever runs Google. I've seen their algorithms change time and time again, and to have one's fortunes rely on someone else's algorithm of the month is a risk. You can create a big community on FB or G+ with a click or two; you can even get thousands of people to quickly press a "like" button; but it's not yours, and each member's investment is far lower (it was just a click, after all).
That said, if your goal is outreach, that's what you need. You need things people can casually join with a click. Your 500K "likes" aren't worth a great deal, but they're easy to get, and you can reach a lot of people easily if you need to.
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