It is difficult to prove causality between declining church attendance and any single factor. We certainly have some longitudinal data on church attendance through individual denominational publications and things like the:
Yearbook of American and Canadian churches [serial].
Reference | BR513 .Y4
You could try to measure catechesis through Sunday School and/or Bible study attendance. Although I haven't looked at the data recently, I recall seeing a number of years ago proof of "so goes the Sunday School, so goes the church" where declines in Sunday School attendance would foreshadow larger church declines. I'm not sure how you would measure a "social justice focus" - individual churches rarely track those kinds of activities in the same way that they do Sunday School attendance. One could perhaps try to take representative denominational periodicals and try to track the percentage of articles devoted to catechesis versus social justice concerns over time.
You are, of course, not alone in investigating the reasons for church decline and there are plenty of books written that attempt various kinds of explanations (and ways to fix it). Here's a few of that seemed to fit:
Finke, Roger, 1954-
The churching of America, 1776-2005 : winners and losers in our religious economy /
Stacks | BR515 .F56 2005
Coalter, Milton J.
The Mainstream Protestant "decline" : the Presbyterian pattern /
Stacks | BX8969.2 .M35 1990
Reeves, Thomas C., 1936-
The empty church : the suicide of liberal Christianity /
Stacks | BR526 .R434 1996
Hoge, Dean R., 1937-
Understanding church growth and decline, 1950-1978 /
Stacks | BV820 .U5 1979