My friend Sheryl did a post recently that articulated a thought I myself have had for a long time, though it goes far beyond life online.
In both my online and offline life, I have a number of different roles I play. There isn't a single one where I can always speak my mind. Ever.
Why? Because it is in my rational long-term self-interest not to. Some of what I have to say goes against my roles in certain contexts. For example, there are some things I won't say to my kids. There are thoughts I won't post online.
And I wan't to be clear: it's not because I'm trying to hide who I am or what I think. It's simply a matter of whether it is appropriate to express a particular thought in a particular place or not. There are plenty of thoughts I won't express publicly that I express in more private, appropriate settings.
That said, when you start feeling constrained about these different roles, perhaps then it's time to re-evaluate the roles you play and to whom. That includes evaluating where and with whom you participate online as well as offline.