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For the umpteenth time: Because Verizon Media is choosing to shut it down (just as they did Yahoo Messenger and Yahoo Groups). No specific explanation was given, but considering the significant decline in quality of content on the site versus the cost of cleaning it up and keeping it running, it's not a surprise (a disappointment, but not a surprise). Do you really think this question hasn’t been asked dozens of times already? One need only review the question list to find the responses.