The first three don't generate a lot of emails, but the fourth is very lively. All of them offer useful information. If you'd like to meet a lot of people using LinkedIn, and read about their strategies for using it, these are good places to do so. There are a lot of confliting opinions offered about the purpose of LinkedIn, and the best way to use it to further one's goals, whatever they may be.
Note that if you post a message to one of these forums and do not state in your post that you are not willing to accept invitations to connect from people you don't know, you will be sent a lot of invitations to connect. Some view this positively, others negatively.
