About unpaid dues (fraude aux cotisations), the highest financial authority (Cour des Comptes) estimated between 6.8 and 25 billion euros fraud annually in 2014, depending on the evaluation method. [0] Then there's dues exemptions. A study estimated 57 billion euros were exempted in 2015, 57% of which was compensated by the State's budget. [1]
We could also mention tax evasion and other schemes, which affect the ability of the national budget to compensate frauds. Despite arguments about the exact number, most agree big corporations pay less taxes than the small on average [2]. It's interesting to note the evolution over time of subsidiaries of CAC40 companies in fiscal paradises. [3]
Overall, there's plenty of fraud and "legal" exemptions on all levels. Quite enough to close the social security deficit.
> Personally I can't wait for France to adopt a fiscal data module like the Belgian HoReCa black-box
I think that's happening. The law is clear that accounting software has to be certified since a few years (which has been a major hurdle for FLOSS projects). Personally, i'm not so much interested in such measures as most of the fraud is committed by the very wealthy and such measures could affect smaller businesses and common people in some ways: need to invest in more expensive accounting equipment/software, more difficulty for undocumented people to find honest jobs (though they'll still be able to work for big industrial groups who are famous for exploiting them).
[0] https://www.ccomptes.fr/system/files/2019-11/20191202-synthe...
[1] https://www.cairn.info/revue-de-l-ires-2015-4-page-3.htm
[2] https://www.liberation.fr/politiques/2017/02/24/les-entrepri...
[3] https://www.capital.fr/entreprises-marches/les-entreprises-d...