I've invested a lot of time into LinkedIn. But the options are limited.
Possibly for finding like-minded researchers to join up with, but our professional organizations all have their own conferences and/or site forums for just that purpose. Relevant news is distributed via dedicated e-newsletters and publications. All hiring is done via specialty pages as well. I occasionally see people from this or that place have looked at my profile, but it's always someone that I've already met with in person, or someone at a place where I've already sent a CV as part of the application process.
My husband is an engineer and works mostly overseas on contract. While he also belongs to several organizations and regularly attends conferences and meetings all over the world his last two jobs came via LinkedIn.
He may meet people at the conferences/meetings but LinkedIn allows them to keep in touch much more easily while connecting to others that they may not have met. His professional contact list is now quite extensive and he rarely has to look for a new job (once the current contract is up). There seems to always be someone he has connected with that are in need of his services. And that doesn't count the headhunters (who seem to be rather worthless when it comes to actually reading qualifications and experience).
Perhaps your profession is made up of a smaller group but for many people who work in a professional environment LinkedIn is THE source and way to maintain connections. In the global environment we now live in it just isn't feasible to make/maintain the contacts needed to get the really good jobs otherwise.
For those who stated that Facebook is just as good, well, good luck using Facebook to further your professional life. It may work for a small minority but the vast majority of professions aren't interested in seeing your personal life mixed in with your professional credentials.
