The part about the private messages and huge media files is not clear. If I want to share some message with my friends only should I encrypt the message using their public keys and publish the result on blockchain? Would those be $number_of_friends messages or is there a way to publish it just once? How expensive that would be? Should I use IPFS for media files? Has anybody ever implemented these parts in practice?
How can I convince people to use my Facebook killer app instead of Facebook if they have to pay for every action (update profile, send message, post something, etc.), the price is not clear in advance (who knows how much "gas" every action would cost), and the process of publishing is not instant (IIRC Facebook has 2 billions of active users, can Ethereum handle that scale? It's supposed to be used by thousands of different projects, right?).
What's sidechain? Is it something I have to develop myself (i.e. something Ethereum does not provide out of the box)? If that's the case why do I need Ethereum at all?