Here are some common answers:<p>- ban the email address associated with an account from creating other accounts, or in the softer form make someone blocking a user on Twitter block every address using that address. This would run into problems with things like + aliases and people running custom domains who have an effectively unlimited number of addresses, all of which can be reduced but not eliminated if you invest time in it.<p>- require every account to be linked to a cell phone number using SMS (“mandatory two-factor for everyone!”). This would be more effective than emails but would cost some users real money, screw or delay others if they have unreliable cell carriers, and completely block anyone who doesn&#x27;t have a cell phone.<p>- require every account to be verified using a credit card, blocking anyone without a card and no doubt discouraging others. This works great for metafilter.com but there&#x27;s also a reason why MetaFilter is a site loved by thousands of users rather than millions.<p>The problem is that Twitter was a VC-funded company which had a big IPO based on highly optimistic growth estimates and their corporate thinking appears to be focused on maintaining those unsustainable numbers. Anything which slows people creating accounts goes directly against the goal of having ever-larger numbers to report.