How do I obtain statistics on openings? I'm looking for statistics on the probability of a given opening being used in a game, and by what percent its use is growing or declining over some set timeframe, for example, over the previous year. How do I do this?
Any chess database (chessbase online, for example) can give you general statistics on how often a move is played in a given position, which you can use to get a feel for how popular an opening is.  To see an opening's popularity over time, and other more sophisticated metrics, you could use a commercial product like ChessBase, which has options like 'Opening Report' which gives you graphs of frequency over time, top player games in the position, and the like.