The posters on here are not over suspicious, the system is.
All staff where I work have been made fully aware that socialising/fb befriending etc of ex pupils is seriously frowned upon, can be interpreted as attempted grooming and could lead to dismissal.
That is the current culture in state education in the UK.
Any other behaviour is seen as professionally risky.
That is why people are telling the OP what they are telling him. He doesn't want to know and only posted on here to boast about how fabulous he is because ex pupils want to socialise with him, under the pretence of asking how many beers to drink (?!)
When I was a student (esp sixth form) it was very different. We stayed in touch with teachers, we went to parties with them, went to the pub with them, got pissed with them and that was the culture back then.
It made us feel grown up and special that real adults were interested in our opinions.
It also lead to two of my close friends having sexual relationships with teachers. At the time they thought they were terribly mature and cool. Now we can all see that the staff in question exploited their position of power and the relationships were imbalanced and very inappropriate.
Most staff would be able to control themselves. Some can't. The guidelines are there to protect students and innocent staff.
It is sad, I would love to keep in touch with some students but I want to keep my career more.