Interesting development. As Ross says:

Jaiku is more than a Twitter alternative. With Google's portfolio, you could easily see it augmenting Gtalk and Orkut on the web and mobile clients.

Robert Scoble explains it this way:

[Why did Google by Jaiku?]

Orkut?

Yeah. Now do you get why they just bought Jaiku?

Now do you get why the world is going to pay attention to what Google releases on November 5?

Yeah!

Facebook has real competition coming. Competition they haven't yet faced.

However, I saw that Scoble had made this post because he tweeted about it on Twitter.

The key question is going to be "which is more important, the technology, or the social network?"

Jaiku seems to have a powerfully interesting product. Arguably, twitter's api has received more uptake. And twitter has more users.

UPDATE

Maybe Zoli has answered the question about the whether Google bought Jaiku for the network or for the technology .

My only question is why Google had to apply it's standard process of freezing newly acquired applications, like they did with Writely, JotSpot..etc? (Existing users can continue Jaiku-ing, but new signups are on hold.)

If Google was interested in the network, my guess is that they would have kept the service open. If I remember correctly, they did not close access to new YouTube users when they bought them.