Do social classes hang more with the same social class? Do social classes hang more with the same social class?

Further, how "reasonable" phenomenon this is?
If we take "social class" to mean income/wealth as it's most commonly the case in the West today, then there's even an academic term for the uncommon friendships across such classes, namely "income-bridging". As for their occurence, let me quote you from a paper by Léna Pellandini-Simányi (which happens to survey Hungarian society, but I think it easily generalizable across the Western world):


  Many of the income-bridging friendships are between former classmates whose class position diverged, yet who kept
  their old friendship alive. 


Let me tell you from personal experience that this is 100% the case for me, if we take the "income-bridging" to mean a tenfold difference in income. 

Also from the same paper on Hungary, this interesting bit regarding the socialist past:


  During socialism, status-bridging friendships were not more frequent in
  Hungary than in capitalist societies, contrary to the image of an egalitarian and open society promoted by political public discourse. [... However] It is important to note that these [socialist-era] studies measured class by education and working position, omitting income.


The last part kinda reminds me of joke/advice along the lines of: don't get a PhD to impress your friends, once you get a PhD most of your friends will have one.