Here’s my take on it and some of what I have experienced, although the experience may well be quite different based on different social connections.
Generally; if it can fit in a tweet, it’s not worth a blog post however if it’s just more than a tweet, it might be worth posting on Facebook.
Tumblr like Posterous(R.I.P.) is for people who don’t have their own blogs or don’t know how to run them.
Each of the other services have different target audiences and different engagement rates at least as far as I have found.
Twitter is typically people who are on to move, connected and engaged, who are actively involved online and quite often work online. Quite often populated by people(Techies) who hate Facebook. Lately however I have noticed a lot more noisy trolling type people appearing on the network.
Facebook from an engagement perspective is typically family, friends, and people who don’t usually use social media too often or know much about technology or the internet. To this crowd Facebook is their go to place for online engagement and the only place you’ll find them.
Google+ is handy for Google Hangouts and connecting with Google staff. There are a couple of communities getting off the ground there but it is still quiet and has offered very little engagement. The automatic photo backup feature is also pretty cool.
LinkedIn; I have found to be a great place to find quality content, but I have experienced very little engagement. Hopefully with all the changes that they have been implementing within the platform, this will change. I personally don’t like the way they handle messages.
At least for me, Facebook and Twitter provide the best ROI in terms of time spent and engagement. Both of these networks have resulted in actual business leads.
Ultimately it would be awesome to publish more on my actual blog, but I almost feel like blogging requires a bit more thought and often this results in me holding off on publishing articles as often as I should.
Optimum scenario would probably be; publish on blog, engage on social network, resulting in people clicking on your profile then landing up on your blog to learn more about you(your business).