I like Assassin's suggestion to take the MB test. Sometimes it helps to know that you're naturally introverted and that it's perfectly normal to have to work harder at making conversation. It's been my experience that introverts are terrible at small talk, but big talk...now that's an introvert's forte, and you can use it to your advantage. You can impress people and make friends out of more interesting kinds of people if your idea of an ice breaker is something like, "So what would you do tomorrow if you woke up knowing you only had 24 hours to live?"

The extroverts may rule the social sphere, but the introverts are the ones that make the world a more interesting place. You just have to spend a little time thinking of unique conversation starters. And don't put pressure on yourself to be a social butterfly. You'll meet plenty of people in your lifetime who will appreciate your quiet, reflective nature.

Now if social anxiety, or depression, is at the root of your silence, or a compounding factor, that's definitely something that should be addressed before you slot yourself into the "introvert" category. Sometimes introverts are really anxious/depressed extroverts.