Third party social media tools really are a two-edged sword. On the one hand, they allow you to get up and running almost instantaneously for little or no money, but on the other hand you have no data security or guarantee of uptime.
I'm reminded of this dichotomy by the recent closure of a number of music blogs by Google's Blogger service. Despite the fact that these blogs were all operating legitimately and within the law, Google removed their content from Blogger without either a warning or an opportunity to back up. This appears to be bad behaviour from Google, but they are not alone. Yahoo! has a track record of closing down Cinderella services without much of a by-your-leave, resulting in confused and unhappy users whose data has been lost forever. And, of course, there was the catastrophic server meltdown at Ma.gnolia, a Delicious.com rival, which resulted in their entire bookmark repository being lost.
Whether it's a company targeting a few users, closing down underachieving services, or suffering massive data loss, there is just no guarantee that the information you put online is still going to be there in the morning.
So does this mean that corporate information should never be entrusted to third party sites? Not at all. Firstly, it's not always possible to run your own internal version of a third party tool, and often it's not even desirable. You could never replicate the networked nature of a third party social network, for example.
Sometimes you can install software, such as Wordpress, on your own servers, but if your IT department is maxed out or uncooperative, you may be forced on to Wordpress.com instead. There could be a significant cost to the business if you have to wait months for your own installation to be set up and for your project to get started, in which case the hosted option becomes the most viable option.
The answer? Your social media tools should, where possible, be regularly backed up just as with your own servers. Recovery of your social media presence should be at the top of your disaster recovery plan, if only because if something serious happens to your company or any of your other data, your blog could be a key communications channel. (This is also a good reason not to host your own blog on the same servers as your main website, by the way! If everything else goes down, you need to have some way to communicate with the outside world.)