If it wasn't for this forum, I wouldn't have known about the offer. 
Some mates have only found out when boarding a bus and the driver has told them. Others didn't know until they then shared that news with them.
They had been using taxis until that point. 

From personal experience, drivers have enjoyed sharing the good news to unsuspecting passengers. It has led to discussions on the bus - usually positive.
However (there's always one of them), it has also led to confusion or resentment for those who have splashed out on a return or day ticket (potentially even a longer ticket), when they would have been better off buying a normal priced single and then a discounted single or the £1 offer doesn't take in to account the significant investment already made in buying the longer term ticket. 

That's where the 'word of mouth' strategy breaks down, that's where passengers become frustrated and that's when additional forms of marketing needs to be introduced and developed.

The marketing that bus operators use, bugs the life out of me.
I would argue that for too many years, it's been ineffective and in some cases now, still is.
It is either too heavily focused on a core route, but ignores the less popular routes. Except if money was spent marketing those less popular routes, they too could become popular.

The 71 is a useful case in example. Constantly neglected, it has timetables and frequency changes that suit the operator (not the passenger), a low PVR, has an underwhelming commercial section bolted on to an underwhelming secured section - but despite all of that, is a key link between towns and villages, it has connections to two national rail stations and feeds in to premium routes at 3 different points on its route.
Why aren't they looking to grow or improve the route? Why isn't there marketing of the route? Why isn't there a push? Why is there a shrug of the shoulders with the standard 'not economically viable' patter.

That route should be popular, it could work better and should have a handsome revenue.
It doesn't and the marketing/commercial teams all have a role to play in that failure.