Since the mid-1990s, libraries have been digitalizing their written and iconographic heritage and publishing it on their websites, portals, or databases. Through online publications, library heritage is being transformed into images or texts that are easily reproducible, transmitted and reusable, thus continuing the process of the "trivialization" of this heritage in novel ways (Jeanneret, 2008).

Since the advent of the social web in 2004, some libraries promote this digitized heritage on social networks. Initially developed for individual and hedonic use and as a communication device for utilitarian purposes by institutions and companies (Mlaiki et al., 2012), Facebook is the social network chosen mainly by libraries anxious to increase their audience (Chevry and Slouma, 2016).

As part of this communication, we wish to question the encounter between social networks and digitized heritage in libraries, based on the following questions:

How do libraries put digitized heritage into circulation through Digital Social Networks (DNS)? What is the role of published images and texts on digital social networks? Does the choice of images and texts reflect a segmentation of audiences? To what extent does the editorial line of the site capture the socio-political issues at stake in the digitization of heritage?

This article will try to answer these questions by crossing the points of view of the two authors, one specialist in the digitization of textual and iconographic collections and their transmission, the other being a semiologist and specialist in the analysis of mediations and their socio-political dimension. The reflections presented in this article are based on the results of a socio-semiotic analysis of Facebook publications of images of the digital heritage of libraries, and of the accompanying texts and semi-structured interviews of the community managers in charge of the animation of libraries. Our study focuses specifically on three different libraries that value their digitized heritage, it takes into account the frequency of their Facebook posts, the size of their community and the interest they can arouse in a public of specialists:

1. The BNF <Gallica: http://gallica.bnf.fr>;

2. Public library of Bordeaux https://www.pinterest.fr/BabordNum/

3. Interuniversity medical library Medic@ https://www.pinterest.fr/biusante/images-de-medic/

After an introduction which will address the problem raised by the valorization of the heritage, the chosen terrain and the methodological tools mobilized, the article will be articulated in three parts.

First, we will present the results of the analysis of the conditions of possibility of the techno-semiotic devices that are the digital social networks, mobilizing a techno-semio-discursive methodology. The analysis of the signs will thus be nourished by the analysis of discourses, crucial to analyze the communicational claim of the devices, as well as by of the analysis of the specificities of the technical device. The analysis of the architext (Jeanneret & Souchier, 1999), of the relationships between texts and images, of modes of address, of markers of authorship (Souchier, 2007), of the small forms that are mobilized, but also of the editorial choices will allow us to show the existence of a dichotomy between communicative and mediation claims (or intentions) (Rondot, 2015).

In a second step, we will show to what extent those signs, through digital social networks, are part of a set of claimed strategies. The strategies of library professionals in the animation of their community will be the subject of the second part of the article. On the one hand, it will be necessary to identify the communication policies of these institutions that value digitized heritage (presence of an editorial line, objectives, target audience, selection criteria). On the other hand, semi-structured interviews will help to better define these strategies.

Finally, from these two approaches, we will explain, how the circulation of digitized heritage on social networks raises three types of issues: the issues specific to the mediation of knowledge, issues related to the semiotic status of digitized heritage, and issues related to legitimizing the socio-political role of libraries.

This research demonstrates the strategic dimension of the digital social networks’ investment: the need to address several types of public, the endowment of the institution with a digital “presence” (image of modernity) and the will to face knowledge mediation issues.

Moreover, it confirms the predominant role of architexts in digitized artefacts circulation, they form and inform the mediation of knowledge by giving it an important communication component.

The investment of digital social networks by libraries, whether there are national, academic or municipal, are always motivated endeavors, influenced by considerations of knowledge mediation. The analysis of social networks allows us to grasp these representations and to see how they are embedded in communication devices.

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