The fact that e-mail is the No. 1 online activity is concrete evidence that there's a huge number of people who feel likewise.
分析一下这个句子的语法结构
The fact 主语(定语从句that e-mail is the No. 1 online activity)is 谓语
concrete evidence 表语(定语从句that there's a huge number of people (who feel likewise. who引导的句子修饰people 也是定从。相当于定语从句中宾语的定从))
收发电子邮件是最常见的在线活动,这一事实有力地佐证了很多人深有同感。