You obviously don't work in software development. Let me give you a play by play of what generally happens in this kind of situation.
1. Someone first has to see the bug. Tweeting/Facebook/etc means nothing. Social media accounts aren't monitored 24/7 and even so, with thousands of people tweeting/facebook messaging/etc, there's a good chance what was sent on there doesn't get seen. The bug reporting system is truly the only way to ensure it gets through, and again, it depends when someone sees it.
2. The bug needs to be reproduced. Without reproduction it can't be fixed. This looks very easy to reproduce so this step likely went quickly.
3. Developers get dispatched to the bug (or since FaceTime is very complex, likely a team) who figures out what is causing the bug. This isn't instant, this can take a very long time to figure out. Reading code isn't like reading a word document and bugs, especially ones involving networking calls can be very tricky to figure out.
4. Once the cause is defined then the scope/impact of fixing it needs addressed. Does the bug ONLY affect Group FaceTime? Does it affect other things? Is it something a patch (which needs developed and tested) can fix? This kind of thing isn't instant either and can take a lot of time to figure out.
5. Once all of the above is figured out then a plan of action is implemented. In this case, Apple decided they need more time to fix the bug and took Group FaceTime down immediately.
You and others need to stop with the silly conspiracy theories already. Educate yourself on how such a thing happens and realize that fixes aren't usually instant.