The difference between the examples given is that Facebook and Linked-In are an invitation-only driven platform. We allow only those we choose to view our profiles.
The BBS here is viewable by anyone. I once belonged to another online forum that was set up this way (free to join) and trolls did some wacky damage, as well as stalking of certain members. One troll even assumed the identity of another member and wreaked havoc with that author's agent. Same troll went online to Amazon.com and wrote scathing book reviews of another author's work.
I do participate openly in another writing forum, but it has a fee to join, which scares away the trolls because it adds a layer of accountibility.
I joined Travelwriters to learn more about the travel writing industry and appreciate the open forum. However, being a bit gun shy from the previous troll experience, I'm not anxious to decloak on a forum that any wacko can access.