There are accounts on Twitter and Instagram that make money by posting sponsored content. Let's say another account does the same thing, but it just copies the tweets/posts from the more popular accounts. Would the more popular account have a case if it were to sue the account copying its content? The smaller account being run from America.
Copyright subsists in "original works of authorship fixed in any tangible medium of expression..." 17 USC §102
Originality requires "independent creation plus a modicum of creativity". Feist Pubs., Inc. v. Rural Tel. Svc. Co., Inc. 499 U.S. 340 (1991)
The creator of a work generally owns the copyright to that work. 17 USC §201
You retain your rights to any Content you submit, post or display on or through the Services. By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license (with the right to sublicense) to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content in any and all media or distribution methods (now known or later developed).
So, you maintain the copyright ownership, and you grant Twitter a licence to do whatever Twitter wants with that work. This allows Twitter to show your tweet around its network and to allow for re-tweeting etc. (Although, it could have done those things with a less permissive licence.) You are not giving other users that licence.