If I were to build a social media app can I use parse?

I can imagine to do that. I would recommend you to go to the site and go through the documentation, so you would see what you can do with Parse.com. Can't say if there are some limitations (or better backend solutions), but basically you need to store data there (on the server - users, groups,...) and then you have to work with them in your code within the actual app.

Parse would be a great tool to make a social networking site.

Parse is a BaaS - or "Backend as a Service." They take care of all the back-end stuff so you as a programmer can focus on the front end development side of things. In your case, they're storing your users, their passwords, and the information about each user.

You wouldn't need to build out a website that's separate from the app. You would just need to build layouts in your app that represents your user pages or user content, then build the java code that will run alongside that layout where you can use a ParseQuery to populate that data by querying the data from each of your users.

It's definitely a lot of work, and you'll need to figure out your data structure and data architecture within Parse itself to find out how you want to do it, but even if it's a lot of work, it's a relatively simple process. It's just going to take a lot of man hours to code it out.

Hope this helps!