From the perspective of the 10% that remain, why would they bother to stay if 90% were cut? Sounds like taking on ten times the day to day workload. In SF, it's not like they couldn't go elsewhere for work.
The people that remain would be well compensated. If you can cut 90% of the workforce and still preserve half the revenue (and that might not be possible), then you can pay each person $1 million per year.
Really though, I just have a hard time understanding what three thousand people are doing to keep Twitter running day after day. I keep coming back to thinking about how Instagram had 15 people working there when Facebook bought them.