In general, whatever you create is yours. Whether registered with the Copyright Office or not.
The advantage to registering with the Office of Copyrights is that your protections may include certain statutory damages, should you sue to to recover damages for infringement.
Without the registration, you can still sue to recover damages, but you will have to prove the amount of damage you are claiming.
In the context asked: You are giving exclusive rights to TFL to maintain your post (the copyrighted material) in public view, in exchange for the ability to post on this site. Quid Pro Quo. In short, it's considered an equitable exchange.
You have not given anyone else the right to use your works.