What is the best service to conduct Focus Groups for game concepts in the US?

Agree on StoreMaven/Splitmetrics, they work well for qualitative analysis (good for estimating and predicting marketing metrics).

If you also want to have a tool for qualitative analysis, pickfu.com is a good, inexpensive option. It lets you show several pictures to a group of people and ask a question (no videos or more complex question structure which is a downside). It's useful only if you want to ask some very specific question to the people – like what do they see on the picture or what they think the game is about based on what they see. The most basic test is only $100 – that gives you a wide audience and you can narrow it down for extra money. The main downside is probably that people are getting paid for their answers, so this might affect their behaviour a lot.

For a more custom qualititative analysis approach, you could design a survey and run the link to it on Facebook for a specific audience. The easiest way to do that is doing a Google form. This way you get more control over your audience + you can use whatever format you like including videos links etc, but it's almost definitely more expensive.

Btw I'd be really interested if you could share some more details about your approach to this in the comments.

I have never heard of anyone doing focus groups for games concepts. If you want to test game concepts, wouldn't you rather just run fake ads with a service like StoreMaven?