Having extremely hard time tracking someone down?

As said above - a very hard time tracking someone down. tried everything from electoral roll to facebook to search engines and nothing. The only thing I came up with was an entry on Friends Reunited but unfortunately it doesn't look like it's been logged into for a few years. The funny thing is when I looked her up on electoral roll several years ago she was on there but for some reason the old entry has disappeared off the register. I am pretty much stumped here. Short of hiring a P.I. to do the job for me what else can I do?