you have to look at twitters history from inception to today.
first off it was hacked together on pretty shitty codebase, which made scaling the service EXTREMELY expensive. they've since moved to different codebase that for some reason seems to be just as expensive to run. even back in the day when similar sized irc networks(to what size twitter was back then) were ran for _pennies_ twitters network was 100x more expensive to run. it's kind of amazing how they managed to do that and not have anyone tell them that their ideas were stupid and that they could have saved a lot of money.
basically, twitter _technical_design_ from day 1 was such that it could not scale to be profitable - which is kind of amazing since there were off the shelf products even back then that would have done it way, wayyy cheaper and with way, way more features.
actually part of twitters early rise was tied solely to american telcos way of screwing over it's customers. namely that you buy text messages as a package _and_ that incoming messages are part of said package, which let twitter send you info kind of free (for them anyways).
I think another thing that happened was that they hired 2000 sales people without thinking what they were going to ask those 2000 sales people actually sell (and if they had something to sell why the fuck they would need sales people to sell it anyways).
another big fuckup from twitter was that they missed their sales window - being unable to scale to profitability would not have been such a problem if they had sold out to ms or someone else 5 years ago, though even then it might have been hard to get a sum out to pay the previous investors - what happened between 5 years and today? well everybody knows already that twitter is just.. twitter. it's not the next facebook - it's the next myspace - and even dimmer buyers know that twitters tech is worth shit ALL NOTHING. for example, if their tech would scale at pennies then all the limits about message lengths and content, client apps and all that would be understandable - but it has all the downsides of a highly optimized system without any cost benefits of such a system.
and well.. another reason to the high headcounts is simply this: the more people work under you the more money you will get paid (out of the investors money). it was just a way to pump out the cash from the sinking ship.