I came across a five-year old article by Lovett Weems in The Christian Century that talks about declining worship attendance. The accompanying graph says a lot, but it’s worth reading the whole article. For example, one explanation for the drop in attendance is aging membership:
Mainline churches have a disproportionate number of members age 65 and older. This proportion will only grow more pronounced as the first of the baby boomers reach 65 in 2011. While it does not appear that death rates are changing dramatically in the mainline churches from year to year, many older members may not be attending as often—for health or other reasons.
The other side of this dilemma is the failure of churches to reach younger persons. This is particularly true for the smaller churches that constitute a large part of mainline denominations.