If it's a third party API key with special priviledged that they hacked, the potential harm is limited.
If they have access to the full system, they could be sending millions of ghost messages to some part of the population right now to get them to do something while we all watch the BTC show:
- scam them
- get them infected to gather a massive bot net
- make them very angry and start some kind of civil unrest in a specific part of the world
- cover a currently happening terrible event somewhere so that we don't learn about it too soon because twitter is the faster medium for that
At this point I realize how critical twitter has became to shape the way we view the world, and govs should worry a lot that this can be happening and act on it quickly.