This is a new phenomena (at least for me). What the rogues are doing is getting into Facebook accounts, I don’t know whether by signing up or by hacking someone else’s. Then they go to Friends ==> Invite Friends and then paste in their spam email list. I know that this must be the case because I have had invitations on addresses that I haven’t used for years but are popular with spammers.
What you receive is a message with the title “Reminder: <some name> invited you to join Facebook” for someone you have never heard of and with a selection of “other people you may know” who you have also never heard of. The problem is that this is a legitimate Facebook email by the time it arrives so pollutes your spam trap system.
The motive? The only one I can think of is a rather elaborate scam involving Facebook reputation. By getting you as a friend they can see your friends. Now that they “know” you they can invite your friends to become their friend and so integrate themselves into your community. There is a chance that they may come across one who is less than careful with their private information and may manage to get into other accounts. Another tack is to use that reputation to send a “Help! I am stranded in Nigeria and my passport and wallet have been stolen, can you wire me some cash to get home?” type of message.