Few people heard about it but everyone saw it. Alegria is the Name of the ecosystem for Facebook Illustrations and probably one of the most copied design ecosystems on the internet.
And there are many reasons that speak for it like, it is inclusive since the people shown don’t are recognizable as humans but too abstract for anyone to feel discriminated against; The style is simple to recreate and to adapt, which saved time and money and; it works really well as a simple vector file, which makes it extremely easy and well performing for web applications.
The downside is that everything looks more or less the same. It is just not interesting it is more like a placeholder.
The actual creator of this style is the LA-based design Agency Buck.