Pixels generally refer to dots on your monitor, though they are also used when discussing PPI (pixels per inch) as pertaining to print resolution for images. For most print projects on conventional press you are looking at images with approx 300ppi. Do not confuse PPI with DPI. Designers tend to use both interchangeably, but they are not. I might print your 300ppi photo on a digital press capable of putting down 2800dpi (dots per inch of ink.) I sure as heck do NOT want a 2800dpi image to do that!
As for inches, picas, mm, cm, meters or Feet, doesn't matter as long as you specify what size your finished piece is supposed to be, have your safeties and bleeds and your crop marks indicated. Though picas are not so much used any more. And any printer can change units of measure to something they like with a simple click pick.
You must use a unit that is constant: inches, mm, cm, feet, miles, whatever...
Pixels are relative, and have nothing to do with measurable distance on a printed page or object. Pixels as a unit of measure can be useful when setting up layouts destined for on-screen viewing and sizing related graphics accordingly. Other than that? Never.
Coming to you from sunny America, where freedom, solvency, and now the presidency,
are all unsurreptitiously counterfeited for your entertainment.
I generally avoid the popular, so-called social media platforms altogether, especially for anything business related. I suspect anyone posting their graphic design work is doing so in search of validation,...