I've come across a few people now saying that the thing to do is choose one thing - Twitter, FB, etc. - and just stick to that rather than trying to do several.

I think that's true - partly because you'll spend more time doing whatever it is, and so get better at what the dynamics are of that space, and so using it advantageously.

Also, you'll just be more visible; if you only post one tweet a day, the chances are that most of your followers won't even see it, so you could be putting in what feels like quite a lot of effort acrosss several platforms, and not really getting anywhere. And something like TweetDeck, where it's easy to post to more than one place is useful, not as carpet-bombing, but, say, to FB friends as well as the much wider constituency of Tweeps. IYSWIM.

On the other hand - networks can create something which is more than the sum of their parts, IYSWIM: I joined Twitter largely to promote the blog, and the blog stats promptly doubled... and lots of people started following because they liked the blog, and there's a Twitter feed on the blog so that my last few Tweets are visible, which keeps it looking lively even if I haven't posted for a while, and so on...

The key is probably to find what few activities work together best like that.

Also what suits you: I love Twitter - the 140k thing reminds me of Sherlock Holmes loving telegrams because they forced you to be concise. How concise before it gets hard to understand... etc. etc. FAscinating stuff on the borders of writing and information theory.