Pros
The people at Social Impact are generally incredibly hard working and smart. They are very dedicated to their work, and this can create a really dynamic and energetic environment. These folks really want to save the world and it's great to feel that in the company culture. They are also very nice and welcoming--this was a small company which grew very quickly. This probably explains the close-knit vibe that remains. Employees hang out socially at great weekly happy hours and other events and are often very close.
Cons
The same things that make it good are sometimes frustrating. Very young company, so there is pressure for you to be willing to work late hours and over the weekends.The nature of the work is very time constrained, so it can be very stressful. Also, despite constant promises of professional development and clear advancement tracks for the junior staff, it can be very tough to actually get recognition for the hard work that goes into the job.