Where can I find social connection events in a city or town while solo traveling?

Couchsurfing, though not as alive as it once was, is still a reasonably good source for local meetups etc. Meetup.com is another, though tends to be for interest-specific groups.

The other approach I have had some luck with is searching Facebook groups for the name of the place/region I am going, and joining some that look interesting then posting to see if anyone wants to meet up, or if there are already some events planned. Eg I have done this with Burner groups in Colombia and Brazil.

It depends what your interests are, and your locations.

Meetup.com - this is a friendly social networking site that covers everything from hiking to knitting to pub-crawling to socializing: whatever. While it's usually for regular members, you can join, attend, and get to know those people - even if it's just for one day. They are already putting themselves out there, so they'll be happy to help you.

Local rags - Almost all cities have a WhatsOn magazine, or like here, portlandmercury.com, where events are held. These almost always include networking or social events in addition to the usual concerts, etc.

Couchsurfing.com - As mentioned already, this is/was a great site that, in addition to staying places, has social events for travelers.

AirBnB.com - Consider using an AirBnB instead of a hotel or hostel. Almost every one I've stayed in has been nicer than a hotel, the same price, and I got to chat with the owners over a glass of wine. They recommended the secret places to go to, and would I like to join them and their friends at a neighborhood potluck tomorrow?

Hash House Harriers - This is a very social and welcoming group that does a casual run/walk followed by some beers and a few traditions. There is one in virtually every reasonably-sized city on Earth. Be sure to ask their contact on the website first what their walk entails. A few are intense with pretty raunchy beer-guzzling, etc., but most are tame, with a few kids ("brats") and people walking their dogs. ALL, however, are welcoming in my experience, 25 countries and counting.

Religious Institutions - At least in the city I live in, if you're at all religious, there are mosques, churches and others that have community events, just browse the websites and resources.

Volunteer Volunteer Volunteer! - This I found to be the most productive and fulfilling way to meet locals and actually some life-long-distance friends. I've spent an evening at a homeless shelter just scooping food, a tree-survey, a turtle rescue, etc. Be polite and sociable, and not too in-your-face about asking what to do, and you'll likely get invited to hang out later or be shown around town.

Go the town/city websites (either official or affiliated), most of them have activity calendars.

Look at local newspapers, billboards in supermarkets/coffee house.

If there are museums or theaters or music halls, have a look at them (on site or web) for activity calendars.

What I have done several times when traveling alone is going on a day trip.

The smaller the group, the more likely you will meet people while on the trip, as you are more likely to really meet people in a small group.

It does of course depend on where you are and whom else will be on that tour.