I have seen many many people say "Thanks so much". I believe what they really mean is "Thank you so much".
Searching "Thanks so much" in this website gives 177 results. But the questions deal with "Thank you very much" or "Thank you so much". See e.g.: “Thank you very much” vs. “Thank you so much”
I personally think it's wrong usage. But I've seen many people use it that way. Is the usage really wrong?
-- EDIT --
@Kevin Workman, I think it's wrong because
1) "thanks" is a plural noun in this context and can't be a verb since it doesn't make sense to say "I thanks ..."
2) I remember been taught that "much" shouldn't be used with countable nouns in plural. e.g.
many friends much money many thanks
But what about "Thanks very much"? Does a different rule apply here?
-- EDIT2 --
I originally thought "Thanks so much" were wrong because the many people I saw saying it were non-native speakers. From the feedback here at E L&U, I got suspicious and asked a couple of native speakers. To my surprise, they both said it's OK. One said it's informal and "many thanks" is too formal, and the other said yeah there is that grammatical issue but it's OK (after I asked specifically about it).
My question is now theoretical and reduces to how to restore "Thanks so much" or "Thanks very much" to a complete grammatical form without omission. For example, the seemingly incorrect "two milks" can be restored to "two measures of milk". What about "Thanks very much"?
@Sven Yargs suggested an interesting theory that "Thanks" is interchangeable here with "Thank you". I am still a bit suspicious though as many things could go wrong with substitutions in grammar.
-- EDIT3 --
The latest development I see in emails is "Thanks much!". This is about one step away from "much thanks"...