In the website http://www.macmillandictionary.com/dictionary/british/thousand#thousand__2, I find that
When you are not giving an exact number, you can use the plural form "thousands" with the preposition "of":
Every day, thousands of tons of waste were sent to landfills.
The words hundred and million work in the same way."
Do "thousands of" and "millions of" mean the same?
I kind of think the number that "millions of" refers to is more than the number that "thousands of" refers to.