Quoted:
Fuck that noise.  500px holds all the cards, and you might not even realize they're capitalizing on your work.

Quoted:
500px account closed.

Flickr may be getting closed at some point soon. Got an email said they sold out to Verizon or some shit.

Quoted:

Not really but their TOS kinda fucks you already.

CONTENT SUBMITTED OR MADE AVAILABLE FOR INCLUSION ON THE YAHOO SERVICES
Yahoo does not claim ownership of Content you submit or make available for inclusion on the Yahoo Services. However, with respect to Content you submit or make available for inclusion on publicly accessible areas of the Yahoo Services, you grant Yahoo the following worldwide, royalty-free and non-exclusive license(s), as applicable:

1. With respect to Content you submit or make available for inclusion on publicly accessible areas of Yahoo Groups, the license to use, distribute, reproduce, modify, adapt, publicly perform and publicly display such Content on the Yahoo Services solely for the purposes of providing and promoting the specific Yahoo Group to which such Content was submitted or made available. This license exists only for as long as you elect to continue to include such Content on the Yahoo Services and will terminate at the time you remove or Yahoo removes such Content from the Yahoo Services.
2. With respect to photos, graphics, audio or video you submit or make available for inclusion on publicly accessible areas of the Yahoo Services other than Yahoo Groups, the license to use, distribute, reproduce, modify, adapt, publicly perform and publicly display such Content on the Yahoo Services solely for the purpose for which such Content was submitted or made available. This license exists only for as long as you elect to continue to include such Content on the Yahoo Services and will terminate at the time you remove or Yahoo removes such Content from the Yahoo Services.
3. With respect to Content other than photos, graphics, audio or video you submit or make available for inclusion on publicly accessible areas of the Yahoo Services other than Yahoo Groups, the perpetual, irrevocable and fully sublicensable license to use, distribute, reproduce, modify, adapt, publish, translate, publicly perform and publicly display such Content (in whole or in part) and to incorporate such Content into other works in any format or medium now known or later developed.

"Publicly accessible" areas of the Yahoo Services are those areas of the Yahoo network of properties that are intended by Yahoo to be available to the general public. By way of example, publicly accessible areas of the Yahoo Services would include Yahoo Message Boards and portions of Yahoo Groups and Flickr that are open to both members and visitors. However, publicly accessible areas of the Yahoo Services would not include portions of Yahoo Groups that are limited to members, Yahoo services intended for private communication such as Yahoo Mail or Yahoo Messenger, or areas off of the Yahoo network of properties such as portions of World Wide Web sites that are accessible via hypertext or other links but are not hosted or served by Yahoo.

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That's basically just allowing them to display the image as you ask them to when you upload it.  That clause doesn't let them do anything else with it.