How good is your contact management system? If six months go by before you realize you haven't seen someone you really like, then (to quote Jeffrey Gitomer) your system sucks.
I think there are very few people out there who have an airtight setup where no one gets forgotten. It's an aspect of networking where I suspect nearly everyone is far off the mark in keeping track of friends, prospects and customers.
Used to be, I thought that CRM (customer or contact relation management) was sad. How could you let a computer program manage your relationships? Foolish me. Keeping in touch with people is good no matter what tool you use.
What does your system look like? We all laugh ruefully along with our "shoebox" and "pile" friends about our own leaky systems, but in the end it's a terrible thing when we realize we've missed some wonderful people for too long.