Neither Twitter nor Facebook are who they are for technical reasons; their popularity was more about timing the market and being the right company at the right time. Unlike say Google, who are on top for technical reasons and dominated by being smarter, faster, and better.
Since their popularity isn't based upon their technology, beating them requires a hell of a lot more than just duplicating their technology.
Twitter is still nascent compared to Facebook traffic-wise, and I bet that the vast majority of twitter users are also facebook users. So, if Facebook gives you the same functionality AND user experience, why maintain your twitter account?
Since their popularity isn't based upon their technology, beating them requires a hell of a lot more than just duplicating their technology.