I'm not sure why people keep saying this, I and a handful of friends, old classmates, and coworkers still use snapchat. I'm not sure if maybe it just depends on the type of person you are or what. I do notice people using it less as they get older, but that was a small handful of friends out of the 20 or so who still use it.
Anyways I think Snapchat is only as good as the friends you have that use it. Instagram for example I can make internet friends/mutuals on, same with twitter, but I feel like snapchat is strictly something for people I know directly, or I'd have to meet the person I'm adding online somewhere first.
I think making friends on social media is similar to making friends in general, whatever you do you just have to be consistent. Regularly commenting on a post, seeing other people in the comments and checking out their profiles and following them, commenting on their stuff, etc.