The biggest question these days is "What is the best way to promote my business online ?" and here are some of the most known options.

Google Adwords is the most known and pretty much the main tool businesses use for advertising. The main advantage of using Google Adwords is having an extremely detailed advertising campaign with lots of options to choose from, especially starting from the last year.
What I also like about Google Adwords is that it spends as much money as you want to, of course everybody has this option now, but spending more money on advertising than you wanted to is not something that will make you happy. You can choose the location, languages and even age, but unfortunately, the people's age that click on your link is not that accurate, at least not as accurate as it would be if you used Facebook.
The bad thing about Google Adwords is that you actually need to watch your Adwords Campaign, otherwise you risk to pay too much or your ads won't show to the right people in the place where you wanted to, it is pretty tricky. Key words are also a problem, some key words are way too expensive or too close to other words that you don't want to be associated with and you get big Bounce Rates because of this.

We just started to look over Facebook as a tool for our Business but I don't know if people want to hear about business solutions (if you are B2B) while reading their friends stories. I think this is an excellent B2C tool but people say that it can also be used as a B2B since you can target, for example, business owners, or you can target people that work in a specific company.
Facebook Ads are very close to Gmail Ads, but when was the last time you clicked an ad when you were reading your emails from your Gmail Account ? Me myself, I can't remember.
Another problem is people's age that use Facebook. I am pretty sure that there are people in their 80s that use Facebook, I even met some, but when you think about Facebook and people that use it, the first thing that comes to your mind are probably people in their 20s, especially in Europe. You might say that you can only target people with a specific age, but if you want to target people in their 50s, you won't find too many on Facebook.
The good thing is that, like Adwords, you only pay for the clicks, so if someone is not interested, it's ok, but if someone is interested you can pay less than you would pay for a Google Adwords Click.
AdRoll

This is actually the main idea of AdRoll, bringing you back what you saw before and what made you a little curious. If yesterday you saw a banner with a nice pair of headphones and you clicked on the banner but for some reason you started to do something else the very next moment, Adroll comes back to you with the same banner the very next day, on a different website that you are visiting. Not only that it reminds you about the headphones you just saw, it actually makes you think about them intensively.
I believe that this is the perfect tool for converting website visits into sales and it is cheaper than Adwords so this can lead to a better ROI.
The main disadvantage is that it might not work that well for a B2B, businesses that sell shoes or headphones will probably get better results with AdRoll than you will.