I don’t think anyone is saying “don’t use the internet,” excepting the OP who asked “where else?” The other suggestions are the “where else”?
Also, it’s pretty easy to focus age groups (and, by default, increase the number of singles). Every house of worship has the “Bible study for young singles” (or some equivalent). I had a buddy who went searching for a “Baptist babe in a tight sweater with a bible cover” and found his wife.
Every “young profession X” group is between 21 and below 40.
In seriousness, the most serious quality people I’ve met in a group, excepting certain combat units, has been the gym at 6:00 am. It’s people with dedication – and a job.
Edit: I would add that a crossfit gym is ideal because of the social aspect.