http://bizzuka.com/company-blog/the-rise-and-fall-of-enterprise-social-networks
I enjoyed this post from Bizzuka (and not just in the self-serving, that's-the-core-of-our-product-positioning way, honest).
We know that ESNs can work - and work brilliantly - but they are pitched as a simple solution, when the fact is they require widescale change in work practices.
Even if the technology seems simple, it doesn't mean adoption is.