I hear this but I'm not sure something will replace MySpace as violently as facebook did. Nothing says they can't come along even though competitors come and go. People still buy things on eBay and craigslist, people still shop on Amazon, people still search with Google.
The big Internet brands have various levels of vulnerabilities/opportunities for competitors. Google search is weakening day by day. Wikipedia's design and software is static. Facebook can't decide whether it's a social network or a news network. Amazon and Ebay are vulnerable to niche competitors. Change is coming, and when it does it's usually unexpected.