Q. Who is the Shema talking to?
Possibly there is a dialogue here, a responsive proclamation. The speaker, Moses, says, “HaShem is our God!” and the people respond, “HaShem is One!” (or, “HaShem the One”).
“One” can be taken in a mathematical sense, 1 as against 2 (making it an anti-dualistic polemic), 1 as against 3 (i.e. anti-trinitarian), 1 as against many (i.e. anti-polythistic), 1 as against none (i.e. anti-atheistic).
Alternatively it means “unique and incomparable”.
Rashi sees a messianic implication in the word “our”: the God who now is “ours” will eventually be accepted by the whole world as the one God of all humanity.