Try messaging someone you used to work with to check in - you may enjoy catching up or even stumble into being able to give or receive help.
Maintaining contact happens rarely. And no, a linkedin add doesn't count at maintaining contact if the last private message was exchanged on the last day of work.
I suspect there's a sort of introvert / extrovert miss match.
The times when I have contacted people, it's been about as well received as I'd expect a contact from a former coworker to be. It's just that nobody ever initiates. I'm not getting anything out of trying to maintain these relationships except stress, so, why bother?
And, now, someone (actually 3 someones, because I was at +3, now at 0) has gone and downvoted my original comment for whatever reason. ¯\_(ツ)_/¯
Edit to add: I have initiated contact before and been well received. Nobody contacts me. It's tiring to have to do 100% of the work to maintain these relationships.