Is it correct to write "Good to hear from you" ?
I have seen it many times but I feel it should be "It feels good to hear from you" or equivalent.
Yes, it is correct.
Like in '(I) thank you', the implied words can be omitted.
It's not incorrect, by any means, but it may be less formal, which is why you hesitate. If something doesn't "feel" right, consider your audience and context rather than focus on what is or isn't "correct." It may be that the sentiment itself is not quite what you want for the situation.
Also, if anyone ever said "It feels good to hear from you" to me, in writing or speech, I would probably assume that one of us had terminal cancer. (which is to say it's both overly formal and overly intimate for most situations). I'd aim more for "It was good hearing from you" or something a tad closer to casual.
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