Social media networks are becoming an increasingly important communication
medium between you and your customers. Like domain names, customers expect
that twitter.com/YourBrand leads to your site. They also expect that
twitter.com/YourRealName is a direct link to you. A competitor having
control of your name on social media sites means they have a direct
communication channel to your customers.
Even if you have a well-established brand, it's important to protect the brand
on social media networks. As you can imagine, the cost of recovering the
brand after someone else already has it (if it's even possible) will be far more
expensive than pro-actively registering it now.