Your email signature communicates how people can get in touch with you. You've become accustomed to making yourself reachable via email, phone, mailing address, and fax. But in today's world, the people you communicate with are likely to be using alternative web platforms for sending and receiving communications or building relationships.
The most obvious social network to include is LinkedIn. Most savvy professionals use the site in some capacity, and everyone's looking to grow their networks. It's the no brainer social link to include in all of your communications because these are contacts that you'd likely love the opportunity to connect with if you make a career change or simply change your email address.
Also, if you're open to more frequent and direct access to contacts, don't shy away from including your Skype name or Instant Messenger account info. You might also link to your own blog or website, if you have content that's relevant to the people you communicate with professionally.
Don't forget that you can create and save multiple signatures with most email programs. So each of your favorite social links don't need to be included in your primary business email signature. Mixing and matching social sites to tailor your signature to your audience is a great way to share different elements of your web persona with the diverse groups of people you communicate with on a daily basis.
With MyBlogLog you can include a socially savvy signature in Outlook, Thunderbird, or Yahoo Mail by inputting your social services and grabbing the appropriate code.
Another alternative to a more social signature can be achieved via some web 2.0 business card services. Meecard, for example, lets you blend together your social profiles and content to take with you as an embeddable email signature.