Start with a pencil and a lot of paper. Before all of that, you have to decide if it will be a general purpose social network or you have already decided to beat us.
So draw the design. One of the reasons I avoid FB if I can is this - bad design. It's not intuitive. Did you choose the database, the stuff you will keep in it? If all that is clear, start with the code. Build a simple web page just for the tests. It has to have all functionality but the CSS is not needed just for the testing. So, build the database and start testing. When all works as expected, do the design of the page. When that task is finished you are ready to launch it. With a few sentences is simple as that. Do you want to run it on Python? Django, Flask or Pyramid are the libraries. Pick one.
So draw the design. One of the reasons I avoid FB if I can is this - bad design. It's not intuitive. Did you choose the database, the stuff you will keep in it? If all that is clear, start with the code. Build a simple web page just for the tests. It has to have all functionality but the CSS is not needed just for the testing. So, build the database and start testing. When all works as expected, do the design of the page. When that task is finished you are ready to launch it. With a few sentences is simple as that. Do you want to run it on Python? Django, Flask or Pyramid are the libraries. Pick one.