Some competitions require entrants to upload their creative works through services such Facebook, Google+, Flickr, Twitter, etc., etc.
One of the problems with this approach to running a competition is that the entrants' creative works are not just subject to the terms and conditions of the competition. They also become subject to the terms and conditions of the social network through which the entrants' creative works are being uploaded.
In addition to being subject to the terms and conditions of the social network at the time of uploading the works, they will also be subject to future changes to social networks' terms and conditions. These can happen at any time and are normally applied retrospectively. This means that it is impossible to be entirely certain about how creative works submitted to competitions will be used in future by the social network and its partners. Even if their T&Cs were fine today (and at present we know of none that comply with the Artists' Bill of Rights) there is no guarantee that they would remain so.
These concerns mean that we cannot recommend any competition that requires entrants to upload their creative works through a social network. Competitions should arrange that entrants upload works directly to the promoters bespoke competition website, this ensures the entrants' works will only be subject to one set of terms and conditions, i.e. the competition organisers. This ensures clarity and simplicity. It is possible to promote the competition via a Facebook page to engage with your community. It is also possible, by means of a customised Facebook page, to display competion entries that are hosted on the competition website.
While we will not go into detail about the various social network terms and conditions it should be noted that they all have terms of service that claim extensive rights to use any content uploaded to them, including passing on such rights to third parties. They all claim more rights than those set out in the Artists' Bill of Rights which is yet another reason why competition submissions should not be uploaded via social networks.
However, the competition may be promoted via social networks to engage with potential entrants, but uploading of competition submissions should not be routed via social network services or file sharing sites.
Another problem with social networks is that many of them actually delete copyright information from all uploaded digital content. Images for example contain metadata, i.e., information about the photograph, such as when it was taken, the lens used, and crucially the name of the person who took the photograph. This latter item, the name of the photographer, is part of the copyright metadata, and usually this will be accompanied by another piece of information, the copyright status, e.g. copyrighted and all rights reserved.
This copyright information is embedded in the photograph, and although not displayed can easily be revealed by a tool such as Jeffrey's EXIF viewer, there is also a help video which shows how easy it is to use. Digital copyright metadata is the equivalent of an artists' signature on a painting, the authors' name on a book, it is an artists' moral (and legal) right to be identified as the author of their work. To remove the authors' name from a creative work is a dishonourable practice, it also contravenes the law in many countries, such as in the USA for example.
Unfortunately the practice of removing copyright metadata is common. For example, Facebook and Twitter automatically remove embedded copyright metadata from images displayed on social network pages. If someone downloads that image or shares it, it becomes an "Orphan Work", subject to abuse and possibly even copyrighting by a commercial entity.
IPTC publishs a survey of social media and image hosting services that remove metadata from images. Please study it. Then read about and support the Embedded Metadata Manifesto. To learn how to participate in the survey that collected the data, visit Controlled Vocabulary and see their survey info page. You'll be doing a good deed.
At present various associations and campaign groups are pressing for legislative action to give a regulatory authority the power to require that an organisation cease removing copyright metadata.