Here's what I've found. My Facebook, Twitter and MySpace sites are nice, but they are social networks, so not really for business connections. You can get a social network that has a lot of business followers, but it takes a lot of work.
I also use LinkedIn, which has more business followers, but it also takes a lot of work to get kinds of followers that want to use your work.
I have a blog that has social and business followers. The one thing a blog does really well, is get it's content onto the search engines. They love blogs. I've posted photographs on my blog, and the next day it's on page one of a Google image search.
Social networking sites, LinkedIn, etc. are nice, and I do suggest having them, but you need to know your clients. Are your clients using these sites? Do they use these sites to find photographers? Will you get a decent return on investment of your time by using these sites?
I also stay active in forums, like this one. I have even received an assignment because someone had seen my posts on a photo forum.
To really be active on all the main sites, you could easily spend 3-5 hours a day creating and posting content. There are only so many hours in the day, and you need to decide where to best spend your marketing time.
Have Fun,
Jeff