January 10, 2020 (updated August 2021):
A number of other places on the web are devoted to Washington, D.C., baseball, past and present. Here’s are links to some of them:
“The Expansion Washington Senators:
https://www.facebook.com/groups/463338547208950/ . Because the page is now private, you need an invitation from a current member to join.
D.C. Baseball History Yesterday and Today:
and the associated Facebook page, Washington D.C. Baseball – Yesterday & Today:
https://www.facebook.com/groups/60009109815/
Washington Nationals History (Current Franchise):
https://www.facebook.com/groups/1592969114297565/
Washington Senators (The Summer of ’69):
https://www.facebook.com/groups/1779972678964841/
A Facebook site devoted to Frank “Hondo” Howard:
https://www.facebook.com/groups/293945135091/
Squibber is the quarterly online newsletter of the Bob Davids Chapter of the Society for American Baseball Research. Join SABR and the chapter to read interesting articles about D.C. baseball.
Suggestions for additions are welcome. Of course, the team’s official web site, washingtonnationals.com, has many useful features, including all-time leaders lists for the Expos/Nationals franchise.
(I’m posting the Facebook links, even though I am no fan of Facebook and have tried to fully delete my own Facebook account.)
As far as I can tell, D.C. Baseball 
History has a strong connection to the Washington Baseball Historical Society. I’m not sure the society is still as active as it once was. Nats News, the quarterly newsletter of the society, is no longer published and its longtime editor, James R. Hartley, died in August 2020. The web site and Facebook page, run by Mark Hornbaker, is going strong.)
Washington baseball history 