I've done a little bit of this. Just like anything else, it can have good results if done properly. If not done properly...well, results are mixed. It depends on what kind of connections they have, and what kind of referrals they may bring to the table.
If a person has lots of connections and is of the mind that they'd rather do business with known associates, then there's a good chance for payoff if they like the service you provide AND have associates whom they can refer. If they don't really care much about where they get their hosting/design/development, and/or don't show a huge history of social networking, you might be better off using another source to build your portfolio...or just write off any work you do for that customer under the heading of "advertising". (Actually, you could do this for all of your portfolio-building clients. Doing this can also give you practice at valuing your services.)
If you choose this route, don't let this be the only way to get your business started. It's certainly one method - but just like any other advertising, you shouldn't just use one form. (If nothing else, if this doesn't work out well, you will still be using other ways of starting things off. One of them might be more effective.)
Good luck!!