If you're only doing one, Facebook is where you should put your advertising dollars in today's marketing landscape. There's a few couple for this:
(1) You need to start building brand awareness, credibility, and a social following. Unfortunately you can't really do this without ad spend because unpaid organic reach is basically extinct on Facebook. Putting ads and boosted posts has the side-effect of growing your social following and reach, which comes in handy down the road for demonstrating credibility, and also gives you a place for your fans to identify themselves—i.e. community building.
(2) Again, credibility is key. You don't want to put an ad in front of someone that just says, "Hey you, buy my stuff!" You need to offer something of value first, and one of the best ways I've found to do that is to build content that is relevant, timely, and provides real value to your audience without asking for a dime in return. Facebook allows you to do that, and to measure it. Then you can retarget those people and follow up with your sales pitch, after they're warmed up. You can use this strategy with Google too, and you should, but I've found it works better on Facebook for most businesses.
Google is great for finding an audience that is looking for your product/service right now. And that may be what you're looking for, for example if you're offering a product that directly competes with a brand that already has a lot of traction, Google ads can help you put your pitch in front of people your competitors have warmed up. For many businesses, though, a heavier focus needs to be placed on building market awareness of the problem you're offering a solution to.
I haven't spent a lot of time with LinkedIn ads, but I will say that Facebook works tremendously in the B2B space as well. The reality is Facebook is where most people, regardless of profession, spend a LOT of their spare time consuming content and it can be a golden opportunity for getting your content in front of the right audience. Anecdotally the only people I know that spend any real amount of time on LinkedIn are recruiters and the unemployed.