Likely one of the most important skills of the present age.
Happen is a great site for those professionals who are in transition. It is an opportunity to develop self-marketing skills and form connections with others who have been down-sized, or whatever. It has a fair bit of face to face interaction.
The Toronto Finance Network is a LinkedIn based group whose goal is to form a fellowship of accounting professionals. Monthly face to face interaction with peers.
LinkedIn is an increasingly important site for professionals to have a presence. It is also a forum of discussion, and a means to have a rudementary web presence, much like a mini personal web page, but within a limited framework. I am open to connections, but have not been a big user of the recommendation aspect, mainly due to the quality of many posted to build numbers. References I can get, and can provide, however, I do not see the need to have them online.
I have a presence on Facebook and Twitter, but use those mainly to keep in touch with people I've known in the past. There are over 350 other social media sites that exist.