> Lately, I've been realizing that even though my coworkers like me, and I like them, they are not really my friends. Nobody who leaves the company stays in touch with me.

I thought this was the norm, not the exception. I'm not in touch with of my past colleagues anymore. Reasons: a) we never hanged out outside the office (besides, beers on Fridays from time to time), b) changing jobs usually means changing cities, and using Twitter/WhatsApp/Facebook/Email is something that I find useless if you want to keep in touch with people; for me it is in-person hang outs or no touch at all.