Here’s why not:
1) No APIs yet, so no limited extensibility and hard to work into my social media syndication strategy – I can’t get my blog to cross post to G+, for example. The xhtml / rel=me thing should work but between my ineptitude at HTML and bugginess in Buzz, I can’t make it work.
2) Not enough people on there that I actually know – the only circles of mine where people are saying anything are “randoms,” “journalists” and “PRs” – which tells you a bit about the early adopters I know.
3) Too many randoms are adding me for no apparent reason – I can understand why some of the big hitters have stepped off the platform within days of joining
4) I don’t use enough of the other Google apps (Picasa etc) that would make it really useful. Maybe I should.
On Facebook at the moment I have one fundamental complaint: the newsfeed’s selection of news stories is currently completely borked. I’m getting old stories from a very small set of my friend-network because Facebook is trying to guess which "circles" I want to hear from and excluding many of my friends.
Sort it out, the pair of you!