Last year's introduction of Facebook into the wild world of stock markets was a bit of a wild ride, as "FB" dropped onto the market at a high near $40 per share and within a few months had plunged to below $10. It has now stabilized somewhat south of $30, but it should (hopefully) be safe to say that investors have learned their lesson about untried web technologies with uncertain revenue streams.
An Initial Public Offering, or IPO, is a company's introduction to a stock exchange. In Facebook's case, most of the recipients of the initial stock offering were the investors that had backed Facebook while it was still private. Some other big funds and banks benefited as well, in addition to the founders of Facebook themselves.
One hopes that investors will be a bit more circumspect with Twitter.
Like Facebook, Twitter is a free-to-use social networking platform that gains its revenue through advertising. Unlike Facebook, though, Twitter doesn't currently have the expandable platform for things like gaming and integration with various third-party applications. Sure, you can link certain games and Twitter syndication applications with your Twitter account, but it's not possible to have the same level of interaction with the cute blue bird as you can through Facebook.
All in all, Twitter isn't likely to jump into trading until late 2013 or early 2014, so you have time to save up for its potentially overvalued stock. It may turn out to be an investment opportunity, but the safest bet for any company like Twitter or Facebook would be to assume that less technologically-inclined investors will drive the price up before the inevitable crash, and more savvy investors (like yourself) can drop in and pick up a few shares at clearance price, and hold them for the long run.
Of course, Twitter could start out low and take off quickly, so like always, you can trade at your own risk, but preferably with someone else's money. After all, that's what all the big banks do.