Why are Millennials leaving the church? It's simple. Mobile social computing has replaced the main draw of the traditional church: Social connection and affiliation.I think Beck's insight is sound. The main appeal of any voluntary organization is the social connection with the people there. If it is to serve a function beyond social connection, then the activity that people do together has to be worthwhile in itself. Facebook can't replace, for example, playing sports together, no matter how much you like the camaraderie of the team - playing the sport requires others, and playing has a value to you beyond the social connection.
So what is the value of the activity of church? The stated goal is to worship God. I think it is a well attested sociological fact that collective worship can be more powerful than individual devotion - perhaps the most powerful of all human activities. But emotionally powerful worship is rare in ordinary church life, especially for young people.
My church is the kind of church that builds powerful social connections from regular, face-to-face interaction. Ours is a small-town church that plays a significant role in our town. It makes sense for us to get together regularly at church.
Most millennials are more likely to go to large, self-contained churches that could be located anywhere. The social space of a megachurch is not that much different from the social space of Facebook. They do not need church to make their social connections, which they then nurture daily by virtual means.
Of course, it helps that the people in my church are so old that most of them have not adapted to Facebook. But that will gradually change.