I can see why. With chemicals, dose can make a huge difference in effects. People are prescribed 5-20mg methamphetamine (Desoxyn) for ADHD, are these people the same as the people bombing 100mg for the huge rush repeatedly? Absolutely not.
That being said, the pharmacology of low dose methamphetamine seems to be much better known than the pharmacology of *repeated* low dose MDMA. PTSD treatment experiments for instance are using low doses of MDMA (50-75 mg) but a finite number of sessions. I see no reference whatsoever to what happens when MDMA is repeatedly administered in low doses. I really can't find a reference to whether tolerance will build up or not. Even in Bluelight, no anecdotal reports exist at first glance.
In short, this is fairly unknown territory. If I had to guess, though, as a pro-social drug it would be something every couple weeks max, just due to the way MDMA works.
A pro-social drug that *only* opened people up a little bit, to ease conversation, that was repeatable, but does not have tolerance issues, or cause sloppiness, deaths, health problems, addiction and other issues, would be quite the hit to be honest. In a way, I imagine this is what the OP is looking for.
Sadly, such a thing does not exist.
It may seem inconceivable to those who can only think of monging out on the typical full strength recreational dose, but MDMA has been mentioned on the social anxiety forums and the like, speculatively, for this exact same reason: low dose for mild social anxiety relief. Generally speaking, for many reasons relating to the neurotoxic nature of MDMA and high tolerance buildup, this is not recommended, of course. But I fail to see failing to see the point. It's just a dream people have, even if it wasn't reality.
If I were to play it safe, I'd probably look at some of the "less" (well, more like unknown) neurotoxic alternatives though, like 5/6-APB and bk-MDMA. And personally I would only use "as needed" which is not often at all, certainly not weekly / regularly. Even a mild effect can trap people -- for reference, look at what happened when social anxiety folks started taking phenibut daily in high doses. But I too have been interested in what a low dose of 6-APB (in particular, due to the nice additional touch of the baby mushroom effects) would be like. Sometimes you just want to do something with just a touch of enhancement instead of a cuddle puddle.