There’s a difference between Twitter account and an Twitter advertiser. None of the advertisers have returned. And, those that continued to advertise in the vacuum… saw their ads run alongside content they’d rather not be associated with and they, too, eventually pulled.
Advertisers are fickle to a point. Given a stable place to advertise, like YouTube, where advertisers can define very specifically where they want their ads shown, they’ll spend buckets of money, increasing spend as the number of users increase bringing more potential eyes that can see their ads. Quarter by quarter, the number of ads certainly flux based on advertiser need.
BUT, provided an unstable place where an advertiser cannot be assured that their content won’t appear beside content they’d rather not be associated with, as we’ve seen, they restrict their spending and redirect it towards more controlled sites. All of the advertisers that have pulled out are not waiting for the number of users to increase, they’re waiting for that certainty of where and when their ads will be placed. Once THAT problem has been solved, then, and only then will they see Twitter as viable again.