[tomtoise]

Facebook app synchronizes with your address book. My best guess is that you've left facebook sync on and forgotten about it, added his number. Facebook has looked up that number, seen it belongs to this friend of yours and suggested a facebook friendship through that.

[talltofu]

Is there a setting for that? I don't remember permitting Facebook to access my personal data. But then again, knowing their constantly changing privacy settings, who knows what they are upto

[talltofu]

I just checked that I had never saved his phone number as a contact - I just gave him a missed call probably so that he could have my number