Very interesting post Stuarty but I guess based on some of the responses its fair to say that experiences differ in some cases.
We use most of the social sites you mention but like Boxby have set up automatic feeds so that our blog updates LinkedIn, Twitter and Facebook.
Measuring and having a clear understanding of the acquisition cost of each new customer is key element in both targeting and setting marketing budgets, and right up there with measuring the lifetime value of each customer, so monitoring outputs from these social sites, when possible, is definitely the right thing to do if your business's web presence generates revenue for you.
We've seen a steady increase in sight traffic since we created the autofeeds and observe a step increase if we post useful (or what we hope are useful) information in related forums or discussion boards and at the same time have seen an increase in our CTA conversions so it works well for us but I can see why it wouldn't for others.
The point you made about people 'going off target' is an important one, especially for Employers. Many large companies simply block social networking sites to avoid this exact problem but if you routinely task staff with engaging in the use of these various social media networks to promote your business as part of their job clearly this won't happen so you need to be set up in a way that will enable you to address issues with them spending more time messaging mates, etc. Employers should make sure staff have clearly written job descriptions and include an internet / e-mail policy in place, discipline and grievance policy, etc in their contracts of employment / employee handbooks.
Yes - I see the irony in my above paragraph
Back on target.........this approach may work for some but not for others but the key thing is to measure / monitor results and continuously review and tweak your approach to maximise your results.