I've been spending a considerable amount of time learning MongoDB as part of the MEAN stack (I'm new to the MEAN stack), and I feel like I'm just barely starting to get the hang of it. Recently, however, my supervisor at work (who's an experienced programmer) suggested I learn relational databases (it could be that it's used by more companies), and I have taken his suggestion to heart since I'm only a junior developer, and would like to move up soon. My personal goal/project is to build a social networking site where one group of users have the ability to search for another group of users (and most likely charge a membership fee). Would the database matter for a project like this? I would love to kill two birds with one stone by learning something (and learning it really well) that could be used in my personal project, and in a professional job.
I'm certainly open to learning more than one language, but I need a starting point, so I need something that will help me accomplish my personal goal. Since I've spent a considerable amount of time with JavaScript (as oppose to, say, Python), having to learn relational databases concerns me. Are there any downsides to just sticking to MongoDB?
Thanks in advance!