I bought Twitter•fm. What should I do with it?

To be honest with you, having Twitter in the domain name is a risky business. They will have the right to take it down at any moment.

I would never use a domain with a trademark word unless I get the trademark's owner permission.

risks aside, perhaps get permission from Twitter to build something that helps them and you can also create a biz from? e.g. .fm makes me think I can tune into specific radio stations made up of a stream of tweets on a topic.

I hope you didn’t pay too much! You unfortunately won’t be able to do much of anything with this without Twitter sending you a C&D.

It's illegal to own a trademarked domain.
You can use this for another type of business which is no way related to Twitter or social media but a company like Twitter is smart enough to mention all type of business categories in their TM Classification.

A buddy had one that included the word "instagram" in it. Instagram sent a C&D threatening 100k fine, so I would be careful.

I don't think there's a lot of value. Twitter is not going to want another company building a service, product, or platform upon theirs that also uses their name. I believe in years past they were pretty litigious when it came to companies building post-scheduling tools and other services that tried to use 'twit' or 'twitter' in their name.