Social media - Facebook, Twitter, Instagram, LinkedIn (the ones I have profiles on). I send out friend/connection/follower requests and hope to see something like 10% follow-back. That is usually about what one can expect.
So, how am I doing right now? On Facebook, the only one with a limit to the number of "friends" you can have, I am just about at the limit of 5000. I've been going through the friends list and removing the ones with the lowest number of mutual friends. First I removed all that had less than 20 mutual friends, then I had to remove all the ones with less than 100 mutual friends. But, I'm still closing in on that awful 5000 limit so now I have to remove anyone with less than 300 mutual friends. It's just plain stupid, Facebook. Why do you do that to us? And to make matters worse you don't give us any management tools for our friends lists. And, when a third-party app comes out for doing just that you make code changes that prevent those apps from working. What are you people thinking? Facebook is the most user-unfriendly social media site of them all.
LinkedIn - I have a hair under 14000 connections and it grows every day. No connections limit that I know of on LinkedIn, thank you very much LinkedIn.
Instagram - the one that grows the slowest of them all. What's wrong with people who us Instagram? Why don't they follow back? I send out follow requests, they're accepted, but the people don't follow back. I sometimes go through the followers list and remove the ones that don't follow back. I have a hair under 600 followers on Instagram.
Twitter - today I crossed over 2500 twitter followers. Apparently, that's pretty good. I don't know, but at least there's no limit, and it does grow every day.
All told, on those social media sites I have somewhere around 23000 followers/connections/friends. When my book is finally released that should be a good place to start.
The next step is to set up a mailing list. That will be here on this website. I'll be doing that in the next week or two, probably.
I use the Strava app on my phone when I go for bike rides so maps of all my rides are available at Strava.