I haven't used LinkedIn for my VA business - yet. But I did attend a Social Media seminar and the teacher said that some use the Groups feature to a) find potential clients, and b) access the future needs of your clients.

It is seems that if you sign up with groups (that tie in with your specialties, you can learn what bugs them and offer solutions and become an unofficial expert on topics)
And if they start to recognize your name they are more likely to turn to you if they need help. Now that was my teacher's theory. One I have not tested. :-)