At the beginning of one episode, Picard referred to Riker (from Alaska) and a historical figure from Ohio as "countrymen." So the United States still exists in the 24th century and includes both of those states.
LaForge was born in the nation of Somalia.
Picard is from the nation of France.
In 2036 there will be a New United Nations, but in 2123 the (old?) United Nations still exists, suggesting they were two separate (perhaps competing?) organizations.
The united earth space probe agency dates back to 2067 and is still around in the mid-23rd century.
In 2150 there was a World Government, and United Earth existed in 2155, personally I wonder if the mentioned World Government and United Earth are the same body.
Earth helped form the Federation in 2161, Captain Kirk referred to the Federation as a Alliance.
I don't consider the Federation to be a nation/state, and I think Untied Earth is probably a international organization that (among other things) speaks for Earth with one voice.
Multiple sovereign nation/states still exist on Earth.