Speaking of lazy : I have your blog in my Google Reader list.
Following is convenient. Commenting requires more clicks…

It’s nice to know someone’s reading, hardly anyone ever comments!!

Also, I’m still trying to achieve some balance myself. I do the cross-posting thing myself more often than I’d like; I can’t always access facebook, though, so there are a lot of things that I just put on twitter, which is as you say a slightly different audience. And then I don’t put it on facebook at all, but by the time I get around to putting something there it’s usually v stale.

Twitter ends up being the most useful way for me to reach a certain audience quickly, I think.

In 99.99% of the cases, the person who posts anything, is only the Nth link in a neverending repost chain.
Tracking down the oiginal source is often tedious.
Sometimes, all the info is provided in the content linked to, and reposting it would be redundant (not to mention a waste of bytes).

The way I see it, it’s all about efficiency :
Someone posts a youtube video.
Someone blogs about this video.
Someone diggs the blog article.
I find the digg link.
I like the video.
If I post about it, the only ‘modern grace’ you’ll get is my not mentioning Digg, and linking to the video instead of the blog.

As far as Twitter vs Facebook is concerned, I only think in terms of audience.
Stuff posted on Twitter is public, and often retweets of retweets.
Stuff posted on my Facebook is private, and clearly linked to me.
Sometimes I post on both, sometimes not.
It depends.

That did not make much sense, but here you go.