From Facebook Developer Wiki

An xid is a unique identifier for a set of comments. You can configure a comment section in your application or site by using:

• The fb:comments tag in an FBML application.
• The Comments Box, for IFrame applications and websites (use fb:comments_(XFBML) to render the Comments Box).

Comments can contain alphanumeric characters (Aa-Zz, 0-9), hyphens (-), percent (%), period (.), and underscores (_) (in effect, the result of any urlencode can be a valid XID).

Facebook recommends you specify an XID with your Comments Box. Otherwise, any extra GET parameters that are added after the URL, but before the hash, will be incorporated into the XID, and the Comments Box will "swap" whenever parameters are appended/changed.