What not have a button to indicate a desire to know if a shared item is in fact true, an invitation to librarians and others to offer more context as it becomes available, flagged later for the curious user?

这句话是对“脸书如何升级自己的用户互动方式”的建议。请问老师们，这句话中的 as it becomes available 怎么理解呢？it 指的是谁呢？此外，flagged 为什么要采用过去分词形式呢？这里是表示被动吗？