I read the linked article and I'm not entirely convinced there is a difference (I've used last.fm happily for five years, and never touched the streaming stuff).
If I've got 30,000 "scrobbles" and all my friends on last.fm, what does like.fm offer me that would make me switch?
They have all this cool data and I feel they're not doing as much as they could with it in terms of modern social features. And for new users I think the experience of finding your friends via facebook is much easier than it is on Last.fm.
But I agree with you right now the site still needs more features. But you can use both services while I work on them. I see a lot of potential for improvement and for advancing the idea behind music tracking.
I am sure they will have FB social features soon enough. Do keep in mind that Last.fm has their own valuable friend graph (even if you may not have a ton of friends on it) that is meaningfully different from the FB graph
If I've got 30,000 "scrobbles" and all my friends on last.fm, what does like.fm offer me that would make me switch?