Answer: Reddit is a social news- sharing service.
Explanation:
Tumblr is a micro blogging and social networking site for sharing text, images, links, audio, videos, etc.
Jux is a platform that can be used to create blogs, create a photography portfolio, and share your thoughts and ideas with other people.
I was not able to find any information on Placasa.
Reddit is a social news-sharing aggregation, web content rating, and a discussion website that is used to share and discuss news. Content such as links, text posts, and images can be submitted that can be voted good or bad or thumbs up or thumbs down by registered users.
Hope this helps! :)