At Twitter Counter you can purchase stats for other accounts other than your own. The statistics include the number of tweets, followers and users followed over time. 

Engagement stats are based on private information that you can only get for the accounts you have access to on Twitter. So if you are an agency and want to get a hold of this information for an account that belongs to one of your customers, then you will have to get permission from them by getting hold of their Twitter login. 

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