Good point. They have already shown good sportsmanship by signing up under real identities, so it's quite possible they'll even extend that to providing useful feedback.

You might even be able to recruit them at some point in the future.

Every touch is an opportunity.

I'd go so far as to see competition doing research as a compliment.

Keep an eye on features that they find interesting. There may be a reason.

As far as reacting to their presence on your service, the golden rule is a great place to start.

Great point. Reaching out to them is an opportunity to understand their use-case and motivations. Perhaps they are interested in some sort of merger/partnership?

A partnership like agreeing on prices for the same kind of service? I skipped my last mandatory compliance training, but something tells me that this might not be a good idea.

A partnership where you send each other customers that are a better fit for the other's product, is the standard level-1 truce here. If a customer calls you ticked off that you don't have some feature, and you know the competitor has it and you're not going to build it, fire the customer at your competition!

There are plenty of ways that companies in the same space can cooperate and compete at the same time without doing anything shady. Most of our partners at autotempest are also technically competitors. But in almost any business there's nothing at all unusual about having a friendly chat with a competitor in the same space. You probably understand each other pretty well! You won't completely open up to them of course, but there might be some info each side is willing to share for mutual benefit.

And you can remind them your TOS that they accepted does not allow reverse engineering.

... but they can download your open source, it's easier anyway