Not in a lot of the bigger companies that I've seen, but my niche is limited and not consumer oriented. I see a lot of companies just barely coming to grips with the potential of these services. Other companies simply blocked them as productivity wasters and moved on.
The company that I work for blocks everything 'social'...Facebook, Twitter, Flickr...which is obviously a nightmare when it comes to following people (e.g. SEOs) and tracking the reputation of the company. Many blogs are blocked too. Getting IT to lift restrictions is possible, but difficult to achieve.
I think that users should have access to social media during the working day, but this should be restricted to before 9am, 12-2pm, 5pm onwards, etc. This would keep a lot of people happy. A lot of people access social sites via their mobile phones regardless, bypassing the corporate network...
This is a tough call. Clearly, employees could waste huge amounts of time on these sites. At the same time, they aren't out there advocating for the company either. Zappos.com has large numbers of employees on Twitter with relatively little oversight and coordination, and has been very successful.
I think to the extent that a company's customers are on these networks, employees need the ability to access them.