Not only are you still using the String class, you are using two different String classes. That really has to stop.
currentLine.reserve(256);
tweet.reserve(150);
Are these really reasonable values?
void loop(){
Ethercheck();
}
Calling just one function from loop() is a waste of time and resources.
Your constant strings in Serial.print() functions are taking up SRAM, too. Stop that by wrapping them in the F() macro.
Serial.println(F("Tweeting ..."));