Someone else hit it. LI will ask people to let it look through their contacts on a mobile device for matches on LI. Depending on how the mobile device is set up, that process sucks in FB contacts and sends them to LI.
The interesting trick lately on FB is the scammers who want more access to your profile so they send friend requests from various innocuous looking accounts, often with the only customization being a photo of a good looking guy or gal doing some activity they've figured out that you like already.
I get at least three friend requests a week from these fake accounts with damn cute women standing around airplanes as their only uploaded photo ever, on a weeks old account.
I wonder how many guys fall for this garbage.
When they first started happening I would search Google for the images of the person to see what the hell they came from. Usually they're some photo of some guy's girlfriend standing on or near his P-51 or some other Warbird, or a local media celeb about to get a ride in one, etc.
The scammers are pretty good at finding these photos and linking them to accounts to send "friend" requests to make aviation buffs, apparently.
It's also interesting how much you can mess with FB advertising by having strange hobbies. They get all sorts of confused and try to sell you some very strange things you don't know exist.