The mutual friend popping up is just Facebook's algorithm working. It gives credence to strong social signals, and the formula for deciding which social signals are stronger than others is proprietary, so I've got to guess about what it defined as 'strong'. It's probably either a new mutual friend between your pal and this person who the algorithm suggested to you, and that mutual friend (so, a fourth person if you are 1, your pal is 2, the suggested friend is 3, this one would be 4) probably has something in common with you but no friends. E. g. it may be someone who went to your school or lives in the same home town or even claims the same religion or devotion to boy bands of the 90s or whatever.
Either way, it's about as random as seeing a pair of people on a city street wearing Chuck Taylors. Random and equally unremarkable.
As for your pal, eh. I find it nuts (I am also older than dirt) that a few months of cordial, playful text chatting is not enough to commit to the incredibly noncommitment-type scenario of going out on a date with someone and, you know, eating at a restaurant and watching a movie or playing miniature golf or the like rather than just doing Netflix and chill.
Seriously. This wasn't a marriage proposal.
You're absolutely right that it desensitizes people, but it also some pretty damned classic avoidance behavior. You text so you don't have to go on real dates with other people. You text so you don't have to strike up conversations with other folks. Etc. etc. lather, rinse, repeat.
I would write him off, personally. He is not going to go out unless it's for a quick roll in the hay. Sorry, but that's what I am getting from that. He strung you along and conversed with you via text message because it was easy and could be hidden if he got a girlfriend or a longer term FWB (he probably had either or both all along, BTW, no matter what he told you).
So! What to do now?
Job One - consider other people. Not just men. I don't mean to date, but to have a social life. A social life does not mean hitting the bars every night. Volunteer at an animal shelter. Join a gym and go. Hit up Meetup and look for groups which interest you, whether it's a Harry Potter book club or a group of people who make quilts or cosplay. Or take a class, anything from ceramics to belly dancing, at your local community college.
Do this not for the specific purpose of meeting men, but because it's an activity you
enjoy. You will meet people, I guarantee it! And even if they are all women old enough to be your grandmother, those woman know someone or other. They don't necessarily have the best ideas of who to present to you as a possible date but it is another avenue to explore.
You can also try POF or the like, what used to be called the personal ads when I was a dating lass. Which is how I met my husband. We celebrate our 25th wedding anniversary next May.
Either way, time to step away from the keyboard.