It's not that kind of product - it's not tooth paste or Coca Cola. Random advertising to random people is unlikely to increase site activity.
Advertising needs to be directed to those who are interested in participating in online communities. For example like some brilliant person managed to do by signing up Codidact in Stack Exchange's free open source advertising: https://meta.codidact.com/posts/280952. But we can't count on free stuff like that, since they are essentially a competing site & unlike us they are in it for the money.
A less intrusive and overall positive way to advertise is simply to add quality content here, then when you are participating in some discussion of that topic on SE, Reddit or similar, link to the Codidact post. We discussed this over at the Software Dev meta: Growing software.codidact
Here is one example from SO when I do as described. This was a programming discussion where I knew I had written a more detailed self-answered Q&A on Codidact. Anyone who comes across my SO post and wish to learn more details can then check the Codidact link. Yet it is on-topic and non-intrusive, it's not shameless self-advertising, which would give us a bad rep.
So find some topic that you know a lot of and/or you are enthusiastic about, write as good Q&A about it as you can here, then refer to it whenever the topic pops up elsewhere.