I whipped up a landing page over the weekend and then had a potential competitor show me an Instagram ad.

Now you have to share the page though ;)

Yeah, this is quite interesting, I've had a similar thing. I've gotten a few weeks into a project where I've tried to find competitors and can't, and registered domains, put up landing pages etc, then suddenly find ads for competitors everywhere!

One theory I had was that it's a bit like buying a car - like you never see that many of your model around and then the day you buy one it feels like every other car is the same model. So really the cars (competitors) were always there, you're just more tuned in to them when you've bought into the model (pun intended).

The other, probably more plausible, explanation is that it takes a few days for the various networks or advertisers and cookies etc to aggregate each persons browsing behaviour (especially new behaviour). So by the time you've been looking for stuff relating to a topic for a few days and then taken a leap they are catch up a little and suddenly able to target you better. I mean, arguably it's still really quick, but not instant.

Something similar to this happened to me before.

I had a startup idea, tried to search online for competitors but couldn't find any, I forgot about it, stopped searching about it, and didn't even start working on it at all, and I had normal ads,
few months later I spoke to someone in real life about it, didn't even open my phone while speaking to him about it, in the second day at night once I opened Instagram, the first ad was about a competitor that had only 10K users