My sim wants to socialize with a sim that does not exist

I think they don't appear in the relationship bar unless you do the introduction interaction. They might talk all night and appear as BFFs, but if they don't introduce themselves they literally don't know each other. 

FlamingoKicker​ In addition to what crocobaura​ said, if you don't interact with the sim on a reasonably frequent basis and your sim doesn't have the Beloved trait, then the relationship with decay over time until the relationship reaches zero friendship and subsequently gets deleted from your sim's relationship panel. 

Hope this helps.

Thank you. I have played the Sims for 25 years and I am still learning things.  I sent the sim to the house where I knew the other sim lived and of course she was there so I had them Introduce each other (they had known each other for a long time) so now they really "know" each other.  So strange.