That is very hard to say. I would doubt very much that it is 50%. Since there are no exit controls in Canada, we do not know how many of these young people return to France after the fact.
We do know that the French community in Canada is growing. Every year we see an increase in the number of immigrants, temporary workers and students. However, I regularly meet people who have lived in Canada for 5, 10 or 30 years and who have returned to France to take care of their parents or something like that.
We do not know what percentage of French return to France, but I think it must be quite small given that economic conditions in France currently offer few opportunities.