Before moving to France in the August 2010, I was already connected already to about 10 social networks, among them LinkedIn. I discovered LinkedIn when working in Los Angeles 5 years ago, and since then enjoyed many useful contacts found through this network.
However, in France LinkedIn is less popular than Viadeo - 5-language social network for professionals. I joined it a month ago and since then was contacted by many experts and even received proposition for joint seminar. Viadeo's structure is similar to that of LinkedIn, even though some features I found less successful. Yet, the most important advantage of Viadeo is that you can find there useful contacts in France.
So, if you are looking for professional networking in France - go to Viadeo.