Why I use LinkedIn
I started using LinkedIn when I left my prior employer as a means to stay connected with my former colleagues. Soon, I shared my LinkedIn profile with new acquaintances, much like exchanging business cards.
I don't accept LinkedIn invites from folk that I don't know. To me, that's merely amassing a group of stuff that has no lasting value. I prize connectedness. For example, as I was flying cross-country, I started a conversation with my seatmate -- five hours of talking about everything. Several days later, I received a LinkedIn invite from her. She wrote, "I'm glad that you and I were plane buddies." Very cool.
I do participate in LinkedIn's Q&A purely for self-marketing. If my question or answer is useful for others, great. However, that is not to say that I submit "whatever." I write thoughtful questions and answers -- it is, after all, self-marketing.
I belong to lots of social networks -- mainly because I'm a social community subscribe-aholic. However, few of my social networks are particularly active. Most of my professional colleagues and friends belong to LinkedIn, so, by de facto, it is where I hang out.