I would be surprised if it has even 1% marketshare on the routes they are on (vs driving, coach, day trains and flights.
For example; in the UK on London -> Edinburgh, Caledonian Sleeper has on average 250 passengers per day (but this is the entire route - not just london to edinburgh). Given there is roughly ~2tph throughout the day for about 16 hours a day, each with ~1000 seats (with very high load factors), that's about 30,000 passengers/day on the "day" train. Probably roughly that again flying. Plus driving and coaches and it is absolutely tiny.
I almost reflexively avoid both intra-European air travel and renting a car when I can. But if you're being cost-conscious, it's hard to avoid the European budget airlines compared to other alternatives a lot of the time, especially if you're traveling between major cities.
For example; in the UK on London -> Edinburgh, Caledonian Sleeper has on average 250 passengers per day (but this is the entire route - not just london to edinburgh). Given there is roughly ~2tph throughout the day for about 16 hours a day, each with ~1000 seats (with very high load factors), that's about 30,000 passengers/day on the "day" train. Probably roughly that again flying. Plus driving and coaches and it is absolutely tiny.