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.
Any layoff is going to be a morale hit, and they're likely to lose some very talented people in the aftershocks. The last company I worked for laid off 25% of the employees, and within a few months another 10% had been poached or found other places to work (myself included).
A friend even tried to fire a guy for bad performance and high management told him to wait for the next "annual layoff", that it would be easier.
A good indication will be if any developers/marketing/sales, etc.. working on core Twitter/Vine/Periscope/Moments teams are let go.
Same thing at Loudcloud, 1/3 of the company that had revenue bearing jobs went to EDS, 1/3 of the back-end developers went to Opsware, and then the other 1/3 (including everyone in the field, all the service delivery/sales teams/support) - were just let go, with very little attention to how good they were.
The difference between these types of these "real" layoffs, and "faux" layoffs, is that in "real layoffs" - companies will try and fight to hold onto the top 5%, but the bottom 95% are let go. In the "faux" layoffs, the bottom 5-10% are let go, but the top 90% are basically okay.
Of course, the reality is that it's never totally black and white, and any real world reductions-in-force are is likely to be some combination of these two, which is why I like the term "Reduction in Force" rather than "Layoffs" - though there are probably some legal reasons why companies use the exact word "layoffs"
Restructures happen all the time, especially at dotcoms. If you work in this industry, then you can expect some level of volatility.
Clearly you're missing @Pinboard from your life.