I think that you need to start with the server, and probably with a question in the B4J forum.
You will need a server with a database that can accept "posts", presumably a mixture of text and image and video content, and organise them into "topics" and "themes". You will not want to send an entire topic to each user every time they need updating, so you will also need a database of "users", what topics they are following, and which posts they have received and not received. Then you have to decide whether the server will notify users that new posts are available for topics that they are following, or wait for user contacts.
After you have designed the server architecture, including the communication protocols that you will use, you can start working on the clients. Will these be Android only, or also iPhone and PC?
I am not sure what sort of answer you were expecting. There are many more choices like this to be made. Facebook has a good head start.