I've quite a few friends who manage and/ or run bars and I haven't heard that many actual proven occurrences of spiking happening.
That said, about 10 years I did spot a man quite obviously pouring a powder into a girls drink whilst she was dancing (she had left her drink unattended). I had obviously been in the drink as I went over and poured the drink on the floor and confronted the fella who had slinked off. As you would expect, the girl and her mates were less than impressed with me until I told them what I had seen and pointed out the bloke. Doormen come over, the bloke goes as white as a sheet (he didn't even try to deny it and it would have been pointless with the amount of CCTV in the venue) and he was 'forcefully ejected'.
Shortly after all the lights went on in and the bar manger said over the pa that all drinks were to be replaced at no charge (I would guess there were 100+ patrons in there at the time). This was in an alternative music venue in the South West.
In the days that followed, I kept my eyes peeled for the spiker, but never saw him ever again in any of the local pubs/ bars/ clubs. The next time I went to back the venue in question I spoke to the security about the incident and I was surprised to hear that the fella unfortunately 'fell down the stairs' whilst being ejected. I was surprised to hear this as the venue was spread over a single floor