I think most people talk about their friends behind their back. However, there's a purpose to it even if people aren't consciously aware of it.
There have been lots of studies about this and it shows that 'gossip' is much more positive in nature than we think. Quite often 'gossiping' about someone's behaviour or something that has happened to them (e.g. gossiping about the friend who is drinking too much, having an affair, or even less important things like being too bossy at the PTA meeting) comes from a real place of concern. By sharing our concerns we close ranks and become a support network for that person.
Likewise, we cement our relationship with those sharing the gossip, as it acts as a barometer for acceptable behaviour and promotes feelings of inclusivity (people like us).
Gossip is a form of social bonding.
In most cases, while it can be a little gleeful at times, it seems it is rarely born of malicious intent. Malicious gossip is, of course, something entirely different and those people are best avoided at all costs.