Yes, there are a few website that are very similar to Musing. The biggest one is probably Quora, but there are also other options such as Stack Exchange, Yahoo Answers or Answers.com. All of these probably have more active users than Musing, but there's one thing that they are lacking; you do not get either crypto or fiat money for asking or answering questions to either of the above mentioned websites.
So if you're looking for another place to ask and answers questions, then yes, there are a few other good options. But if you want to get rewarded for it, then Musing is unique!
Yes, there are several other websites similar to musing.io
Where you ask questions and get the answers to the question.
I was once a frequent Quora contributor.
It pretty much does the same thing musing does, except that it doesn't reward you as musing does.