it is because someone gave it to you(gifthttp://www.chess.com/home/gift_subscription.html) or you won a special tourney.
you are very lucky and you are not platinum but diamond. enjoy it!
Membership?
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Nice, good job. Use this 1 year wisely. Train hard, and make good use of your free membership!
I'll take that membership if you don't want it 
But seriously, you ARE very lucky.
And btw, if you don't know all the features, they're on this page:
http://www.chess.com/membership.html?c=navbar
Have fun with all the fancy features
I just noticed today that I am a platinium member. I probably should't be complaining but I never paid for it and I am confused why I am one. Is this some type of promotion?