LinkedIn is useless...

... if you don't know what you want to get, or what you want to give away.

Some people may want to get:

• a job
• a better job
• a very specific job in a specific industry
• a consulting contract with a specific company

Other people may want to give away:

• their experience and wisdom
• their professional expertise
• their political connections

Of course, the above does not preclude benefits that a person can get via serendipity.

But it's always better to have a clear goal, and LinkedIn doesn't seem different from any other tool: success depends on knowing what we want to achieve, and then using the tool specifically to achieve precisely that.