All of your company's social network accounts (including but not limited to Facebook, Twitter, LinkedIn, Github, Confluence...) must be opened in the name of the company and therefore belong to it, as foreseen in the shareholders agreement. Always use and favor company professional email addresses (as opposed to personal addresses). The same goes for the source code, the brands or the domain names which are to be considered assets of the company so they cannot be claimed by some shareholders as their own property.
However, if the company is incorporated late, the source code and any other filed intellectual property rights must be brought into the company's assets by specific contracts: to that end, please contact a lawyer.
Some difficulties and conflicts may arise when a shareholder leaves or joins: it is therefore important to know what belongs to the company. If you are lucky: the shareholder considered has always shared with you the credentials to access the accounts; then, you will just have to deal with the property of the source code, Github specs and brands, by referring where appropriate to the existent agreement.
But what if this shareholder has a monopoly over these credentials and refuses to provide them? Unfortunately this might lead to dire straits and you might have to go through hardships to recover the credentials: bringing the case before the civil courts, seeking compensation... Moreover, French courts do not consider social network accounts assets of a company.
The legal documents proposed by alf provide for the automatic transfer of source code, brands and social network accounts with the incorporation of the company on the good faith of the shareholders, but this remains a moral commitment, interpreted very strictly by the judges. You should also foresee a contract to transfer intellectual property rights as soon as the company is constituted – the longer you wait, the higher the risk of dispute.
For further information, you can read our blog post on that topic.