Ebola can spread only after a person begins to show symptoms. And that takes ~21 days. Or putting it straight, for it to spread from one person to another it would take 21 days of period in between getting infected and spreading it to others.

Though the growth of number of patients with Ebola is definitely exponential. Please note it takes 21 days for the each multiplication to happen. It started in March and has been been spreading since, 10K is pretty large for a 7 month period.

>>but the sky isn't falling just yet.

In things like this you won't know when the sky will start falling apart. And generally when that happens its already too late, and you would be staring at a pretty big damage.

No body knows how many people have been infected, or will be infected in the next multiplication.

Which is why WHO suspects ~1.4 million could get infected by January. By then it would be too late.