A friend was concerned about LinkedIn stealling email addresses. I have been a on linkedin for years and not had a problem with stolen email addresses. At the time I joined you had to explicitly upload your email address book or enter your gmail userid and password to give them your emails. I imagine that is still the case today otherwise how could they get your address book.
Additionally once you are a member you can adjust your bacon preferences (bacon is like spam in that there is more of it than you want but it is coming from a service you signed up with) to reduce or eliminate notification email. The same is true for Facebook.
I have used linkedin in the past mainly as a souped up address book for my contacts that is automatically updated by them when they move or change jobs. If friend is not getting my emails then sometimes a message via Linkedin or Facebook will reach them when my real emails have not. I have used the search feature sometimes to find a friend of a friend with a certain skill or interest with some success. I also know people who like it's lists feature and the job advertizement feature. When I did more sales I would also use it to research background info on a lead or to get a referral to them. When I was running the CFUnited event we used Linked, Facebook and Twitter to promote the event using the events feature and fan page, with some success.
I am now exploring more ways to use LinkedIn. 
View Michael Smith's profile
I used to get invitations to joint some other social network (Plexo) but I got tired of them and I think I joined the network and then opted out of all the bacon. Worked for me. Alternatively when I have been getting more bacon than I want I have set up an email filter to automatically file the emails in a subdirectory so that I don't have to read them (unless I get curious again). I know this is easy to do in both Thunderbird and gmail and I imagine most other email programs let you do that do.
