I've had a couple of incidents where my seatmate's idea of socializing was to promote their religious beliefs to a captive audience. Luckily, I travel with headphones and have politely disengaged and watched a movie or listened to music. I still greet seatmates, but I'm careful not to give the impression that I'm there to listen to what they have to say for hours.

I had one flight where the person behind me was either to timid or maybe really interested, and the sales pitch from his seatmate for his brand of religion went on for over 2 hours. Again, headphones really helped, but I did hear bits and pieces between songs.