We are onto number 3 of this link.
Generally, ambiverts aren't the greatest conversationalists. Although as the above statement suggests when a topic that an ambivert knows a bit about comes up then they can talk about it for hours.
I remember I was at a party and a couple of my friends were talking about cricket and were about to start an argument with me about it when they realised that they were not even going to get remotely close to winning so they decided against it.
There are some people out there and I can think of a couple already who can talk about anything and literally bullshit their way through a conversation about any topic. I sometimes look at these kind of people and get a little bit jealous became they seem to be holding court but in reality probably no one is listening. So why are you jealous of him or her?
They say that you have two ears and one mouth for a reason so don't worry if you don't have a lot to say by listening you might learn something and at the very least by not saying anything you are showing that you are not an idiot. People who talk too much can show that they are idiots and it doesn't reflect well on them.
When a topic that you know comes up, by all means show your full voice. When you have nothing to say, don't force yourself. You will be the better for it.