I noticed an interesting thing with a certain visitor after posting a blog post to Facebook.
The IP address that the Facebook crawler uses to visit the site on callback to get meta-data has a custom IPv6 sub-range.
IPv6: 2a03:2880:ff:1a::face:b00c
Host: fwdproxy-prn-026.fbsv.net
Notice 2a03:2880:ff:1a::<strong>face:b00c</strong>
How this is done
The IPv4 addresses could only use regular integers (decimal); numbers between 0 and 9.
However, IPv6 can use any and all hexadecimal; 0-9 and a-f.
This means that when allocating a sub-range, they chose the face and b00c as they are all greater or equal to a and less than or equal to f.
Closing remarks
If you know about, or see other companies that are doing something similar, please tell me about it!