Hint 1:
Don't only try 'direct' searches. Look as well for people who might know the person you're looking for. For example, you might be seeking Jane Smith. There are thousands of people with that name, so it will be hard to find her. But if you know that her husband is called Ethelbert Smith and that her brother is called Marvin Hagmeyer you might have more success looking for either or both of them (and then asking about Jane).
Hint 2:
If you know that a person you're looking for had lots of family members in one particular area of the country (even though your friend might have left that area many years ago) try writing to the local newspaper, asking if anyone can put you in touch with your friend. (You can get contact details for most newspapers simply by using Google).
Hint 3:
Try Google. Although your friends might not be on, say, Facebook, they may be mentioned somewhere else. For example I was seeking someone on behalf of an elderly friend. I only new their name, that they'd emigrated to New Zealand and that my friend had got to know them because they both had English Setters. Googling the person's name, together with 'setter' quickly produced the required contact details. (The person I was seeking was on the committee of a society for setter lovers).
Hint 4:
While many (most?) people now mark their electoral registration forms to indicate that their name should not appear on the 'edited' roll, plenty of people still allow their details to go onto it. Subscribing to 192.com will allow you to search the whole country:
http://www.192.com/
Chris