I'm afraid the social media and blogs are here to stay. This is a different scenario than during the 2000 tech dotcom bust. Back then a lot of people were still not connected to the web, and investors realized that paper launches are not guaranteed.
Today however, between mobile phones, tablets, and WiFi, everyone has access to instant social media and blogs anywhere. Further, unlike the internet's first six years of main stream use between 1994-2000 when people were limited to wired access inside, today's sites are all intertwined with connectivity and instant linking anywhere there's a WiFi or cellular signal. Combine that with all the advertisement that can be linked, and it's the next step in information/entertainment evolution, and it's here to stay until the next big thing.
Is Tumblr like Twitter? I fail to see how that could possibly be worth $1 billion. And how the hell does Yahoo even have a billion dollars? As with most Yahoo moves, this has fail written all over it..