Once you have found the person you were looking for, see if you share common acquaintances. Such acquaintances could recommend you directly or allow you to mention their name, making it easier for you to get in touch, particularly if the person you are looking for is highly solicited.
Always explain why you are getting in touch, personalize your communication as much as possible and offer your help in return.
Keep the intermediary in the loop, so that the person you are contacting feels more bound to reply.
Don't forget to thank the intermediary afterwards and to keep them informed of the outcome!
Our tip: LinkedIn has a “getting introduced” function and Xing shows the people standing between you and the other person, as a suggestion of how to get in touch with them.
Getting in touch directly
People are more likely to accept one's attempt to get in touch directly on social networks than in real life. You can take advantage of this possibility when you don't have an intermediary. As this means of communication is not widely used yet, you will increase your chances to stand out among other candidates.
Moreover, contacting somebody on a social network gives them access to your public information, allowing them to get an idea of who you are, to learn about your interests, your background, etc.
Getting in touch through a social network has another advantage for the person you are contacting: they can choose when they want to read your message and reply to it.
Our tip: It is common practice to let the person know how you found them on a social network. Explain why you are getting in touch and the why they are of particular interest to you. In other words, explain your motives.