I think it's somewhat a must read for any MMO developer. Of course it's just one source, but right now I don't know a better book on mmo dev (please feel free to suggest) and was somewhat an eye-opener to me.
(It sort of helped with non-mmo game dev as well).
Of course nowadays people talk about "social mmos/games" (whadda hell those are anyway ) and this book isn't directly about that stuff. When the book was written, FB was not in every mom's computer.
Anyway. Nexic of course has much more experience on MMO dev than me so I'll just leave this out as "my 2 cents".