Referrals only come if you ask for them. Referrals from people you know are the best kind of referrals. Giving referrals goes hand in hand with asking for them. If past clients have a business or provide a service that is outside of your line of business or even if it is loosely related, it makes sense to send a referral and to have the client mention your name when doing so.
You have not asked directly for a referral, but you have helped someone you know in increasing business by sending someone. Maybe it is a car repair shop. Maybe it is a hair stylist. Maybe it is an appliance dealer. Maybe it is a lawn care service. Maybe it is to a factory worker's brother's business. You just never know who may ask you for advice. And you may never know who will ask someone else for advice which could mean a referral for you.
Facebook is a networking site where you have contacts with friends. And those friends can benefit by you referring business to them. The reverse is also true. One quote I use quite often is from Jeffrey Gitomer. "All things being equal, people want to do business with their friends. All things being not so equal, people STILL want to do business with their friends." That is true on so many levels. Gitomer goes on to offer the following: "You don't need more techniques and strategies. You need more friends."
Of course, there are a number of other sites where you can develop relationship such as Linkedin and Plaxo. But the point is that you need to look at giving, or adding value, not just asking or taking. There really is no secret to using social networking sites.