I use the Address Book as a digital Rolodex.
Instead of limiting it to names & addresses, etc., I make a "card" for odds & ends of useful info.
Example: Toner cartridge numbers for printers.
Use the "Note" area for specifics. Use "Urls" for favorite online sales sources.
Obviously, one has to make "Group" collections or individual choices for "real" email use. But the convenience of finding basic info beats hunting for the document or folder or whatever else you might need to get it. (And for heavens' sake, DON'T let Facebook have access to your address book.)
#2: And while we're in the Address Book, remember that you can click the address and have the choice of "Map this address," "Copy mailing label," or "Copy map url." Be sure to click the field label ("Home" etc.) for these choices.
"For what is age but youth's full bloom,
A riper, more transcendent youth" - Oliver Wendell Holmes