Right now +1 is a miniscule percentage of all social clicks, so it can only grow. But if your site is only seeing a small number of US clicks and a high number of international clicks, then unless you are doing something dramatically different than your competitors (i.e. other serps results for your searches), then they are probably seeing the same thing. This is assuming that the demographic group that buys your product tends to have similar behavior across all sites selling your product. This would not be true if you are doing something to influence this result, like selling a product and only offering US delivery when everybody else offers international delivery, but presumably you would know about something that you are doing that would influence results in this way.
I don't think the threat of not using +1 is being demoted in SERPs, I think that it's more like a missed opportunity. As more and more people use +1, search will look more like facebook ads and searches. Meaning that anything your friends like (e.g. have +1'd) will appear first in searches. This will push down other things from the results, including you if you don't offer +1.
So +1 is not about geotargeting at all, it's about friend and affinity targeting and predicting your interests. That transcends geography.
An example would be, you search for "best surfing in spring". If Hawaiian and Costa Rican beaches are all the top results now, but most of your friends live in San Diego, your results might be some San Diego or Mexican beaches because these are what your friends have all +1'd. What will be interesting is how they filter it. Which is to say, will the first 5 results be affinity and the next 5 be organic, or will there be no limit to the number of affinity results. Or, will affinity results be sectioned off in some way, as Google Place results are now. OR will it be a seperate tab, like MAP, WEB, VIDEO are now? But video results are already included in the mixed search results too. So there are many possibilities here, some having more impact than others.
Exercise: In Facebook, type in a common name or product or rock star. The first results will be your friends or fan pages, the next should be friends of friends, or the fan pages your friends like.
Conclusion: Even if you haven't adapted Facebook because you feel it offers you little, it's models of social interaction seem to be becoming standard.