Do you really wonder what Friends character you are similar to?
Does posting your rapper or porn star name on Twitter to participate in a hashtag something that will bring more value to your life?
Don't think I am judging as I do plenty of things online that are a waste of time. The issue with many of these challenges is the information you are giving out. Many of the components of these trends and challenges are information that is used as security questions at many websites.
While some of these may be innocent fun it is also possible that a hacker who has some of your information can then scrape Twitter or Facebook and use this additional information to gain access to various accounts.
Whatever you do online take a moment to think of the information you are sharing and how that could be used by someone with the wrong intentions.