This answer assumes competence on the part of the photo lab you're using. If you think they're not competent, then use a different photo lab instead, rather than trying to work around their lack of competence - because if they're incompetent in downsizing images, they're probably incompetent at other things as well.
Facebook's primary business isn't driven by image quality, so they've chosen to apply heavy compression to any JPEGs they store and serve up - this has advantages both for Facebook (lower storage costs, lower bandwidth costs) and the majority of Facebook's users (quicker download times, lower bandwidth costs). Sure, there's an occasional outlier1 who cares more about picture quality than the average user who'd like higher quality JPEGs, but the folks at Facebook think they'll make more money overall by heavily compressing JPEGs. And frankly, they're probably right: they know this market better than you, me or anyone else here.
On the other hand, a photo lab's primary business is based on image quality. Still assuming they're competent, they'll put a reasonable amount of effort and money into ensuring that image files they're given will turn out as high quality as possible, because that (and cost) are what distinguishes them from the competition. Hence I'd assume that they're going to do well enough on downsizing images that I'm not going to worry about it - but I fully admit I'm not making prints and trying to sell them for real money. If you are, you may want to take extra care on this.
- Note that users on Photography Stack Exchange are very definitely not an unbiased sample of Facebook users when it comes to this metric.