On a related note; Facebook seems to use all manner of metrics to suggest friends. Here's a view I have been able to pin down:

- common IP addresses (this occurred after about a week of common IP address between two accounts)

- constant geolocation commonality (as above, two accounts with no links, two different IP addresses but geolocated to the same place)

- having a certain number of groups in common (unsure of this one, but it is the only link I can make for one suggestion)