# Recommendations for Different Tasks Based on the Uniform Multimodal Joint Representation
## 1 Introduction  
Content curation social networks (CCSNs) are booming social networks where users demonstrate, collect, and organize their multimedia contents. Pinterest is a typical CCSN. Since its inception in March 2010, Pinterest developed fast, broke through the 10 million user barrier [1] in January 2012, and CCSNs have gone on to become a very popular social network worldwide. In China, there are many Pinterest-like networks such as Huaban, Meilishuo, Mogu Street, and Duitang online from 2010-2012. The rapid growth of CCSNs has attracted much attention from multiple research fields, such as network characteristic analysis [1, 2], user behavior research [3, 4, 5], social influence analysis [6], link analysis [7], word embedding [8], search engines [9], user modeling [10, 11, 12], and recommender systems [13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]. As is well known, CCSNs are content-centric social networks [10]. Different from user-centric social networks, users on CCSNs pay more attention to the contents that users collect, which are not only communication carriers but also carriers of user interests. Taking one of the best known CCSNs, Pinterest, as an example, a \"pin\" is an image with a brief text description supplied by the users. A \"board\" is a collection of some similar style pins. In other words, the pins are curated into \"boards\" by categories [19]. On CCSNs, the collection of a user is composed of several boards, and a board is composed of pins. The relationships between users, boards, categories, and pins are shown in Figure 1. A \"user\" represents the users on CCSNs. A \"board\" represents a container of pins and is organized into different categories. A \"category\" is the category of the board, which is given by the user. A \"pin\", which is created by a user, is the basic unit, composed of an image and a corresponding brief text description. Users on CCSNs can \"follow\" the users they are interested in like on Twitter or Facebook. \"Re-pin\" is an action like a \"repost\" or a \"retweet\", whereby users can re-save pins and re-organize them with new descriptions and new categories to their own board. \"Create\" is similar to \"post\" on Twitter or Weibo, allowing users to post their original contents on CCSNs. From the figure, we can see that the pin is the basic unit in CCSNs. Besides the content collections, there are also abundant social behaviors on CCSNs. Users can follow other users or other users' boards, users can also re-pin other users' pins and collect them into their own board. Furthermore, the re-pin path is recorded in CCSNs. All the users who have re-pinned a pin can be connected using a re-pin path. All users of the same re-pin path have collected the same image, but they have organized them into different boards and different categories. As shown in Figure 2.  
On content-centric CCSNs, most user activities are related to the pins. [PERSON] et al. [25] found that only 30% of pins are re-pinned from their followers by statistics on Huaban. Furthermore, users do not follow the users from whose boards they re-pin the pins [4]. A non-trivial number of pins are collected from non-followees [5], and those from native followees are more than those from cross-domain followees [3]. These observations suggest that social relationships are not the main motivation of content discovery on CCSNs. On the contrary, user interests represented by pins play an important role in user behaviors on CCSNs. It is possible that content-based recommender algorithms will be more effective than social behavior-based algorithms such as collaborative filtering. Inspired by this, we managed to implement recommendations for different tasks based on identical representations of pins. The problem can be broken down into two questions: how to represent a given pin effectively; and how to implement the different tasks with the obtained representation. Figure 1: Items on content curation social networks (CCSNs) and their relationships.