A significant problem with social networks is that many of them actually delete copyright information from uploaded digital content. Without this metadata information (and a watermark), images that are shared and re-shared, ad infinitum, across the web become "orphans", i.e., losing all ability to have identifiable ownership.
Images contain metadata, i.e., information about the photograph such as when it was taken, the lens used, and the name and contact information of the author. 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. (You can view a help video that shows how easy it is to use.)
Digital copyright metadata is the equivalent of an artists' signature on a painting or 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, at best, and contravenes the law in many countries. Below is an excerpt from Title 17, Chapter 12, of the US Code related to Copyright Law.
§ 1202 . Integrity of copyright management information
(b) Removal or Alteration of Copyright Management Information. — No person shall, without the authority of the copyright owner or the law —
(1) intentionally remove or alter any copyright management information,
(2) distribute or import for distribution copyright management information knowing that the copyright management information has been removed or altered without authority of the copyright owner or the law, or
(3) distribute, import for distribution, or publicly perform works, copies of works, or phonorecords, knowing that copyright management information has been removed or altered without authority of the copyright owner or the law, knowing, or, with respect to civil remedies under section 1203, having reasonable grounds to know, that it will induce, enable, facilitate, or conceal an infringement of any right under this title.
Unfortunately the practice of removing copyright metadata is common. Twitter and Instagram automatically remove embedded copyright metadata from all images. You can expect nearly the same from Facebook. The result is that when a user shares or downloads your image, it becomes an "Orphan Work", a work whose author is unknown. There are millions of orphan works created each day in this way and they are subject to considerable abuse. In the UK, it is now possible to use these orphaned works commercially. The US is considering ways to do the same.
At present, various associations and campaign groups are pressing for legislative action to give regulatory authorities the power to require that OSPs (Online Service Providers) preserve copyright metadata. You can help. A first step would be to support the Embedded Metadata Manifesto!
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.