I have been using Flickr for years and have really enjoyed it. After discovering 500px and the quality of some of the pictures posted there, I have been spending more time with it than Flickr, but I think the new Flickr App will encourage me to change that. There is just not enough time in the day to take in the beautiful photos that are being shared on the web! Without considering the App, Flickr does have some advantages over 500px as there appears to be more dialogue among Flickr members than 500px members. This is especially true when one joins Flickr groups such as the 100 Strangers Group where group members are encouraged to comment on posted photos in an effort to help each other become better portrait photographers. Please check out my 100 strangers set.
The features I like about the Flickr APP is that one can post photos from an iPhone / iPad camera, camera roll or pictures from photo stream which have been taken with ones DSLR. Posting from photo stream is a feature which appears to have recently disappeared from Instagram. Also the app provides custom filters, like Instagram does which can be used to enhance ones photos. Here is a link to the app write up for a more complete description.