The idea behind Google's 'Social Search' is pretty simple. It is that you trust the opinions and advice of people you know more then strangers on the web. Google takes the information you have already given it and then searches your 'social circle' meaning the people you follow in various services and the people they follow (just one 'step' past who you follow [does not include the follower's follower's follower's]), for an answer to your query. This will become a lot more clear after watching the clips (if you are pressed for time skip the first).
Useful -> probobly.
A bit creepy -> maybe, but Google is just organizing the information you have put out there.
A 'shot' at Facebook -> definitely. This was one of Facebook's 'Ace in the holes', that you would trust your friends more then a general web inquiry. Facebook is still inaccessible to Google, but Google has just planted their flag in a place Facebook probobly felt they would have real difficulty achieving.
Videos embedded below.