From Facebook Developer Wiki

How do you delete a cookie?! I tried with a past time for the expires parameter but it doesn't work.

[edit] Expire Cookie

I have not even been able to get the cookie to have a different expiring time besides 0. If you try a different time such as:

$facebook->api_client->data_setCookie($user, 'connexID', $row['id'], 300, );

The new information in the $row['id'] will not be overwrote. So if one time $row['id'] = 'a' and the next time $row['id'] = 'b', if you have a different value besides 0 or in the expiration time, the cookie's value will not be overwrote.

[edit] Important Note

THESE ARE NOT HTTP COOKIES!!! They are actually stored at Facebook server-side. You can also query the FQL "cookie" table to get them.

It's not immediately obvious when reading this, so I thought I'd mention it here.