I often wonder whether these "layoffs" aren't actually layoffs, but simply performance based assessments. It's not like Twitter is shutting down an entire office, or abandoning some technology, and letting everyone associated with that office/technology go - presumably they are being selective on other factors as to who they let go - and I'm guessing that performance is likely a key factor.
If, over the next year, twitter doesn't hire back that 10%, or hires employees in different technologies/positions (I.E. Web developers instead of thick client developers, sales people instead of developers, etc...) - then this is a layoff. But, if headcount returns to the same number, in roughly the same job areas, then this is just a performance based annual rank/yank process.