I've never heard of these particular Baptists.
Perhaps you have?
They are the Northern Baptists from the pre-war split. It's a large denomination, but nothing like the Southern Baptists. The SBC has done a better job of infiltraing the north than the AmBaptists have done in the south.
Here are some numbers:
American Baptists claim about 1.5 million in the US in just over 5800 churches, and about 3.3 million worldwide in 6500 groups.
That's roughly 1/10 the size of the SBC.
Stats are drawn from Adherents.com
Many American Christians that I know who go to the Baptist church admit they have heard of the ABC-USA. When you talk about "American Baptists" they automatically assume you are referring to the American Baptist Association (ABA).
In 1998 it was estimated there are about 29 Baptist organizations in the United States and the SBC is the largest one. Some that are familiar to evangelical Christians but not wider Christendom include General Association of Regular Baptists (GARB), Independent Baptist Fellowship of North America (IBFNA), General Association of General Baptists, etc.