With so many social networks to choose from their are some do's and don't's you should pay some attention to. Most if it makes perfect sense, but it's hopefully worth repeating.

Don't just join a social network for the purpose of dropping a load of links to your sites/pages. - This is often seen as spamming.

Do take part in discussions that you can contribute to. Be active and share your knowledge.

Only leave links to your site when relevant. A typical example of this is just replying to a thread, page, comment, etc, in order to leave your signature link behind.

When finding or joining a new network, don't aim to add all your links in fifty posts in thirty minutes. Some networks see this as spamming and will delete/lock your account. Do visit daily for a few weeks, look around the forums, and drop a couple of links a day or session if you have many links you wish to promote.

There are some networks that are quite active and don't care about leaving your links. Even myspace or hi5 will let you, if you know your thing. Try not to reach the limits everyday and you'll most likely be fine, also make your messages look as natural as possible.