All of the sites I have used it on are also utilising forms of social media marketing off site to draw people in, so it already has an audience that are familiar with these sites and provides a way for them to interact with them without leaving my site. So in a way the audience that does use it is pre-qualified.
I have the welcome popup active on some of my bars, as I want to direct them to the bar, now this is slightly intrusive and is the only thing I have on there that I would think could cause a negative reaction, so I would consider leaving this off the bar for some sites. The bar if designed to fit your sites colours can also be easily overlooked (hence why on page buttons are also needed in my opinion), that in itself is indicator that the bar is not effecting user experience in a negative way. You can adapt colour, apps and placement, so you can adjust it to suit your site and audience.
It works for the sites I have installed it on, but of course the effectiveness does depend on your audience and their current usage of social media networks. My customer base is 20-65 but from the small amount of data I can gather, it has been people aged 20-45 that have used the bar to recommend pages. This is on commercial websites such as e-commerce, service based and media launch. I would not just blindly install it on all sites though, I think it is still a judgement call.
If in doubt and you have the facility why not ask your visitors, let them tell you whether they want it or not? Social media provides a great way to interact with your audience for this kind of thing, giving you the opportunity to enhance your service excellence and plan your business strategy based on feedback received.
Hope this helps
