Answer: Hierarchical diffusion
Explanation:
Hierarchical diffusion describes a situation where information is first shared amongst certain people such as a group of connected individuals and then those ones spread it as well to others.
This is what happened here. President Obama's tweet was first seen by people who followed him and so were connected to him on Twitter and then as they retweeted it, more people saw it and retweeted it as well leading to the tweet being shared to millions.
