I still maintain that you can’t waste time, ever! If you want to do it—if it makes you feel good and if you aren’t hurting anyone—do it. Is it the best use of your time? Again, only you know that.
Does an online involvement keep you from meeting someone who could have been your life partner? There is no way of knowing. But you can say that about any event in your life. You went left instead of right, so you missed what was on the right. Except, since you went left, you can never know if there was anything on the right.
Does an online involvement keep you from doing other, more “constructive or productive” things? That is, I believe, a peculiarly American concern. It arises from this Puritan notion that we must be busy doing good work all the time. Well, I think a case can be made that establishing a relationship with someone you care about is a pretty good piece of good work. There is a fucking lot of sadness and loneliness in this world.
So, even if it were possible to waste time, I don’t think an online crush is a waste of time. There’s a lot to be learned there. There are serious feelings. There are relationship issues to work through. It’s not a waste of time no matter what standard you apply to it.