Here is our answer. We have found it in the Odoo 9 code. It is surely not the best code, but it works for our needs at this moment. Thanks
<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>
<template id="website_lapagept.website_layout_iv" name="Main layout - IV"><!DOCTYPE html>
<html t-att-lang="lang and lang.replace('_', '-')"
t-att-data-website-id="website.id if editable and website else None"
t-att-data-editable="'1' if editable else None"
t-att-data-translatable="'1' if translatable else None"
t-att-data-view-xmlid="xmlid if editable else None"
t-att-data-main-object="repr(main_object) if editable else None"
t-att-data-oe-company-name="res_company.name">
<head>
<meta charset="utf-8" />
<t t-if="main_object and 'website_meta_title' in main_object and not title">
<t t-set="title" t-value="main_object.website_meta_title"/>
</t>
<t t-if="main_object and 'name' in main_object and not title and not additional_title">
<t t-set="additional_title" t-value="main_object.name"/>
</t>
<t t-if="not title">
<t t-set="title"><t t-if="additional_title"><t t-raw="additional_title"/> | </t><t t-esc="(website or res_company).name"/></t>
</t>
<meta name="viewport" content="initial-scale=1"/>
<meta name="description" t-att-content="main_object and 'website_meta_description' in main_object
and main_object.website_meta_description or website_meta_description"/>
<meta name="keywords" t-att-content="main_object and 'website_meta_keywords' in main_object
and main_object.website_meta_keywords or website_meta_keywords"/>
<meta name="generator" content="Odoo"/>
<!-- OpenGraph tags for Facebook sharing -->
<meta property="og:title" t-att-content="additional_title" />
<meta property="og:site_name" t-att-content="res_company.name" />
<meta property='og:url' t-att-content="request.httprequest.url_root+request.httprequest.path[1:end]"/>
<!-- OpenGraph pour le forum -->
<t t-if="main_object and 'plain_content' in main_object and main_object.plain_content">
<t t-set="og_description" t-value="main_object.plain_content[0:500]"/>
<meta property="og:description" t-att-content="og_description" />
<meta property='og:image' t-att-content="request.httprequest.url_root+'logo.png'"/>
</t>
<!-- OpenGraph pour les produits avec une description de vente -->
<t t-if="main_object and 'description_sale' in main_object and main_object.description_sale">
<t t-set="og_description_sale" t-value="main_object.description_sale"/>
<meta property="og:description" t-att-content="og_description_sale" />
</t>
<!-- OpenGraph pour les produits sans description de vente mais une description -->
<t t-if="main_object and not 'description_sale' and 'description' in main_object and main_object.description and not main_object.description_sale">
<t t-set="og_description" t-value="main_object.description"/>
<meta property="og:description" t-att-content="og_description" />
</t>
<!-- OpenGraph pour les images de produits -->
<t t-if="main_object and 'image' in main_object">
<meta property='og:image' t-att-content="request.httprequest.host_url+website.image_url(product, 'image_big', None if product_image_big else '300x300')" />
</t>
What do you mean by Facebook Open Graph ? Is it means that you want to add a button of Facebook to share that product page into Facebook ? In this link (https://www.odoo.com/blog/odoo-news-5/post/top-apps-themes-in-odoo-store-314) there is 4 buttons ....Please confirm that do you want exactly like that ?