What legal requirements are there to create a Social Media WebApp?

You need to work closely with a lawyer on this. Legal fees should be part of your overall budget. I do this kind of legal work. The list of your legal needs includes working with a lawyer on selecting a name that can be trademarked, then registering trademark on that name. Another area includes patenting your app or licensing use of the needed patented items. Then another big area of law is privacy law. How does your app affect privacy of the users and/or non-users? The other laws depend on what your app does. These may include copyright, royalties, and on, depending what your app does, where it is sold, and who will be using it. You will need a good Terms of Service, written to minimize your liability and increase user awareness. You will definitely need to have a lawyer with you every step of the way in this process, as well as all the time you have it out on the market. With most internet sites and apps, there is legal fine-tuning that must be done on a regular basis. For example, the Terms of Service will need to be rewritten regularly. You should have all correspondence of a legal nature going through a lawyer so they know when to update the TOS.