Speaking of the About page, it should list where the data came from.
Another helpful feature: a generated list of the companies for which you have the most data for, so that seeing the list is just a click away rather than having to type it in.
edit: the source information is mentioned on the front page: Salarly does not rely on self-reported salary data but uses the salary data of foreign workers in the US. This salary data must be reported to the Department of Labor and is available on their website.