@stanrobinson I would agree that it would be easier to get people to your Facebook or G+ because of the pre-exising audience vs. getting someone to your own website.
That said, I strongly stand by having your own platform separate from social media. You never know when things are going to change on social media. Just think about the whole Facebook Edgerank that now makes you have to buy advertising if you want to reach your own fans with a wall post. Or how many businesses relied on being able to redirect visitors to a welcome tab on their page who have lost a lot of revenue and ability to build likes thanks to the timeline change.
Plus, if you build up all of your content on a social network, what happens if your account gets canceled because you broke a rule and didn’t realize it? If you have content on your own blog, you can have the security of a backup and just start a new domain from scratch. If it’s all on a social profile, it could be lost forever.

What I tell clients is to 1) always, always, ALWAYS control their own space, i.e. their own domain name and hosting account, and let everything else point there freely… and 2) It’s great to have FB fans and Google circles, but those guys have plenty of traffic, they don’t need our help…
What we’ve found most useful in SM to date is updating groups in Linked In (via Hootsuite), when we have a post that will appeal to that group. We’re careful to take the extra few seconds to “check” the right groups for the right posts (i.e., new writers won’t care as much about a book marketing post and authors don’t need beginner editing advice), but that does bring a lot of new blog subscribers to our door… and it’s efficient — I can go interact with those groups when we see activity!