If I you were given a billion dollars to disrupt Facebook, how would you go about it?
Clarification: Your objective is to get enough FB users onto your platform over a 2-3 year time frame that more or less will lead to FB's obsolescence. This is intentionally vague to encourage creativity and also question what it is that keeps FB users on the platform in the first place.
Buy/cultivate influence with sympathetic publications/journalists or create/acquire your own media presence and just hammer them with every negative thing you can find about the management of the company, the business, the users, the content, anything. Pay moles inside the company to leak sensitive information to you.
Then when/if you manage to bring Facebook's reputation down enough, target advertisers with a coordinated campaign to boycott this "horrible" company.