Twitter does not need to gain new users - it needs to reactivate old users. The statistic I cannot get past is that they have lost one billion users. That is a much different problem than most companies are dealing with.
I honestly wonder how many of twitter's accounts are fake accounts, it seems that spam is a huge problem there.
Probably quite a few, but no matter what an inactive account is -- spam, a novelty account, or an actual inactive person -- if you have 3 times as many inactive accounts as active accounts, there is something awry.