Despite the superficiality of a question about intros, there's a deeper question about how common are the various ways of responding to "How are you?" in a neutral fashion. Just answering if people use it is barely touching the surface.
Learners of English are sometimes surprised by this greeting: "How are you?", from now on 'HAY', seems almost intrusive into one's feelings and situation, expecting a detailed retelling of one's activities. All that is really doing is saying 'hi', you aren't really expected to go on at length, just say any one of the many short empty responses. "Pretty good!" "I'm doing well", "Fine, you?". No one cares about the answer.
OK, that's harsh. People of course care, but it's just a greeting a start of a conversation, not a literal, prompt for an essay. You're not expected to be factual or detailed, just respond to get to the next thing, if there is even a next thing (there's the classic Britishism "How do you do?" whose most expected response is ... the same phrase "How do you do?"; illogical if you're are forcing literalism, but just fine with a curtsy.
With that out of the way, one can put some minimal meaning into the response: life is good, life is bad, or somewhere in the middle, the "so-so" area. First, the most common response is on the good side because anything else almost requires an inquiry into details and the HAY phaticism is, given the above, not expected to elicit that.
There are many responses to HAY (the following is in no way exhaustive)
- good: I am doing well, I am good, I am fine, I am great, Doing well, I'm excellent!, 'Great. And you'.
- bad: Not so great, I could be better, (a sigh), I've had better days, Fine considering,
- and in between: I'm OK, So-so, Fine, I'm doing about average, fair to middlin (a quaint understated regionalism, don't use it it's just for exposition)
People use and recognize 'so-so', but really not as much as other instances of similar meaning things. It used to be more common but not as much nowadays.
There are a lot of caveats about this graph. Among other problems, Google NGrams is limited to print, not how people actually speak, I may have left out more common ways of writing these things and subtle variations on spelling can sway things considerably.
The word is just introduced in EFL situations, in a misguided attempt to understand the greeting literally, in order to fill out the lexical gap ("there's good and there's bad, what about in between?").
Socially, you should almost always say one of the 'good' ones, unless you're really looking for sympathy and expect time to explain.
Of course if you're dealing with a health professional, that is one instance where you want to respond more exactly with the literal truth. "How are you?" "Not so good, the steering column has me trapped and I can't feel my legs" is very appropriate (if that is indeed the case).
'So-so' is only a response to "How are you?" and "How's it going?", but not really to other greetings like "What's up?", "What's happening?", "Hey", "Good to see you!". As I speak mostly with AmE speakers, I don't really know the greeting habits of outre-mer colinguates, but Google ngrams tells us that there has been a slight preference in the US for HAY in comparison to the UK, but I don't find that difference significant.
'meh' has been suggested as an alternative to 'so-so'. Grammatically, 'meh' is more of an interjection than a consistent part of speech. You can't really say "I had a meh day", you're more likely to say "It was really meh today". The meaning of meh almost is an abbreviation of "I don't really care", or 'I have no feelings one way or the other", and doesnt work very well as an adjective. Also, to answer HAY with meh is a little inappropriate as a politeness response.