Social: As we might expect for a revolution in religious thinking, it started small. When Mormon says they were "not half so numerous" as the people of God, he is describing the beginnings of the group. Remember that this is also a phenomenon that begins with the children who did not remember the covenant, and certainly would not have begun with all of those children.
What is clear in this verse is that the movement did not remain a "child's movement." The "dissentions among the brethren" can only mean that there were problems within the people of God, and that many of them began to follow this different religion espoused by the children. There is something about the religion of the children that is becoming attractive.
If we remember that this is a process that has happened before, and that previous versions of the apostasy carried with it the trappings of wealth and social distinctions, we can more readily see that rather than a simple ideological question this was an issue of the type of society that they would be, with a significant number apparently envying the lifestyle of the "others" - those who are generically labeled Lamanites in the Book of Mormon even if they were not literal descendants of Laman.