FB and LI save us the time and pain of updating our contacts phone numbers and email addresses. If you send someone an email and it fails to arrive - because your contact has changed their address, simply check their FB or LI page and copy/paste their new address into your email tool. Your email tool will remember the new address.
Facebook has replaced the friends and family portion of my phonebook.
If you don't have an account on FB and LI, then your business suffers a hit on credibility. It's like not having a phone number or email address. If you give out business cards, or use email or have a website, then it may also make sense to have a Linked-in page and a Facebook page (for you and your business).
Linked-in has replaced the business portion of my phonebook / rolodex.
If you want people to be able to find you, then it's a good idea to have a Linked-in page.
The warm-and-fuzzy feelings (and laughs) that you get from keeping in touch with friends and family are their own reward (even if all you do is read their posts and look at their photos).
When you come across folks who say "I don't use Facebook or Linked-in" - it's because they don't care about reason #3 (the cool factor), or they have privacy concerns or time management issues or they have a bad habit of posting and then regretting it, or they are outlaws on the run, or maybe they don't have a computer or have some sort of physical handicap that makes it difficult.
If you care about that person, politely inform them about reasons 1 and 2 (increased credibility and eliminating the maintenance task for contact-info).