I know facebook tracks how users browse their site and which profiles they look at, how often and how long (from articles facebook staff were interviewed in). I wonder if they have starting using that data to recommend friends? If someone is searching for a particular person frequently enough, or regularly visiting the public parts of their profile, they probably know each other.
- 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)