These guys under reported Model 3 sales by a large margin, but if you take the 6000 number for May, it slots in with some well established cars.
May 2018 YTD U.S. Passenger Car Sales Rankings - Best-Selling Cars In America -
If Tesla does manage to get to 5000 cars a month soon, with most M3 sales in the US this year, that will put it well up there on the best selling cars list for the entire year. The M3 is priced in the premium market, but its base price overlaps with the top end of the mass market "family" cars like the Accord and the Camry.
The Model S collapsed the top end large sedan market which wasn't big, but has been dominated for years by European brands. The Europeans have taken notice and are doing more than anyone else to electrify. Part of that is Tesla breathing down their necks, but it's also their governments encouraging it.
The Asian and other American car makers have been more complacent, refusing to quite believe Tesla could eat into their market share. But a successful M3 would do that. That's probably why the FUD machine has gone into overdrive. Some people are trying to kill Tesla before Tesla can kill their favorite car company.
Tesla also has a big time advantage. There is a lot of talk of "Tesla killers" from one brand or another, but nobody is going to have enough batteries to compete in numbers with the Model 3 for at least 2 years, and probably more like 4. Look at how much Tesla evolved from the first year of the Model S in 2013 (about the same timeline point in MS production as M3 production now) through 2017. Even through 2015 the Model S was a dramatically better car than what they were building in 2013. And the Model X came out during that time frame, much later than expected. The Model 3 was much closer to its original timeline prediction.
There are other car companies that will be coming out with EVs in the next few years that might compete with Tesla on features. They will struggle to match the convenience and quality of super chargers, and nobody will be able to compete in volume. Which enables Tesla to build EVs cheaper than anyone else.