I am a diocesan priest in the parish of Cyrill and Method in the Archdiocese of Vienna, and a teacher at the Check out this post by James Chastek on interpreting religious statistics. He makes three points: (1) An evaluation of religious statistics that looks only to the last 50 years is short-sighted in comparison with the long-term nature of movements in religious convictions; (2) people leaving the Church is an ambiguous statistic; it could be a sign of a spiritual good, namely a greater appreciation that belonging to the Church and church attendence should be connected with the truth–having previously accepted the Church not as true, but simply as a part of culture; (3) the statistics often rely on non-objective methods to determine the numbers of members of the Church; e.g., simply asking them whether they are "Catholic".
I have often made the second point in response to what I sometimes see as an exaggerated concern with statistics of church membership, expressed on the occasion of hearing the numbers of persons leaving the Church. While it is better to be a Catholic and live as one than not, it is also better to be honestly not a Catholic than to be dishonestly a Catholic.
One must admit, however, that while cultural christianity never saved anyone, it can be an occasion for a real encounter with Christ, who is the Savior of all men.