As you can see, a single source/person is the main reason for the massive jump in revenue for the foundation. Now, where did the $3,550,000 in grants go? well, pretty much all to the 501(c)4 March For Our Lives Action Fund (which is allowed to engage in more political activity and lobbying than a (c)3):

A 501(c)(3) organization must operate exclusively for a charitable purpose. This could be poverty relief or civil rights advocacy. It could be carrying out the work of a scientific journal or educating students. The entity might set out to prevent cruelty to animals or to advance a religious mission. The law also forbids it from engaging in campaign work for or against a political candidate. Some (nonpartisan) educational initiatives—such as sponsorship, debates, or voter registration outreach—are acceptable. “Insubstantial” political advocacy is also acceptable, as defined by the relative portion the organization spends in relation to its whole operating budget.

A 501(c)(4) nonprofit operates according to its social welfare purpose. It can be a local association of employees or an organization devoted to the promotion of public safety, recreation, or some other social welfare mission. If it primarily advances the interests of a private group of citizens rather than the common welfare, it will not qualify for 501(c)(4) tax-exempt status.

The most notable difference between these two types of organizations is that under the (c)(4) provisions, this entity may engage in lobbying for or against laws critical to its members and mission, as long as its political activities do not comprise its primary function.Therefore, a group that wishes to participate more than incidentally in political activities will opt for 501(c)(4) status.