Should we tell people Lemmy exists

Tell them, sure. Preach it to them, a little less so imho. I often see people being such a pain in the you know where insisting on making other people using whatever it is they're using, or proving them wrong when they don't want to. It's not the best way to make it attractive to anyone.

or is the organic slow growth much better long term?

There is no assurance it will get better. The reality is that it can fail, like it so frequently happen with anything 'organic'. Living things can grow for sure, but they can also not grow at all and die. For many reasons, including the lack of care and... excessive care too (try giving too much water to a plant, or too much food to some creature).

What I do is tell people wanting to reach me I don't use centralized social media at all. And that they can find me around here (or on my blog, or through email).

A few will indeed reach out to me using those, while most will simply not bother trying. Quite normally, I would say, as most don't care about reaching out to me specifically (or anyone else, for that matter), they care about getting an answer (from anyone) to whatever is bothering them, or getting instant feedback/validation. And they care about it as easily as possible, putting in as little efforts as possible... meaning they're not looking forward to trying new and less popular ways.