When you have a social website or a website where you can register, would you eventually delete them after a certain time (after a year of inactivity) or would you rather keep their account records for ever?

I know websites like Facebook have large amount of inactive, duplicated and fake accounts. So I'm wondering if after two years of inactivity it would be alright to send the account a warning email of deletion unless they log in.

Just thinking of a clean and efficient database management or any implications this may cause to new potential users.