Yes, anime fans are shy in real life because unfortunately anime and manga is not a popular topic that you can talk casually about in public. Not being able to talk about it makes us look like we have nothing to say, and in turn that makes us look like we are shy. That can't be changed unless you have friends in real life that are also Anime fans like you and they know what you're talking about. Of course it's not like that only for anime fans, there are many specific fandoms and people belonging to that fandom would find themself to be in that situation, like book readers or soccer fans if you are in united states, because in real life you cannot choose what kind of people you are going to be facing in everyday conversations.
However, a forum on the internet is widely frequented by people of the same interests, so you don't have to be afraid of talking about those things. What comes after, is that you also don't have to be afraid to showcase the things that you like in your avatar and signature since you have that ability. Not to mention that some people find it simply more comfortable and/or use that as a way to advertise their interests. For me, it's no longer that I can't show my irl picture to people since I can and I did (on another forum) but it's that I like to use my avatar/signature and showcase different things, things that would interest Anime fans rather than pictures of myself that would not allow me to show "how" and "why" I am a fan of something. Seems like a basic concept, really.