It's pretty hard to work in those clubs without brushing shoulders with them, but of course saying hi to them in a nightclub is obviously a different degree of fraternization than running a casino for them, etc.
The issue of his gangland connections was discussed in the latest book on FS---written by his long time valet---a very perceptive man.
It seems that Frank's attraction to the mob was not b/c they rubbed people out---but b/c they were skilled businessman, and knew how to make money, especially in Las Vegas and Hollywood.
The valet wrote how people in the entertainment business gravitated to the mob for financing projects b/c they took the financial risks Wall St would not.
That was then. It's a lot different now.
Not to mention orgiating and swapping poon.