Who infiltrated linkedin site ?
LinkedIn was manipulated by a group of intruders that created a breach in the system. In this time, two other sites were accessed illegally and 8 millions of passwords had been stolen: Last.fm and eHarmony. The intruders didn't find the usernames so they have just the passwords of the accounts.
The security staff found out about this leak and they say that the principal intruder can have access to the usernames. If he doesn't do anything with them, the breach can be used by other "PC users" that know how to operate with it. The site's security is now weak, so if you own an account you should change your password really fast.