What's the max. length of a Facebook uid?
I've seen multiple sizes and I don't want to waste server memory on a MySQL field which is reserving too much space for too many characters. What's the biggest they can get and will this ever change?
Answer
This is how integer overflows, integer->string migrations etc happen, by making datatypes too restrictive. Splash out on a few bytes for a 128b varchar and save yourself the hassle down the road. If your user base gets so massive that you need to be worrying about how many bytes you will save by crunching data types of UIDs, consider yourself a huge success and that is a problem you will be happy to solve.
Short answer, I dont think anyone will be able to answer your question, "ever" is a long time and who knows how many entities facebook will have enslaved by then.
I'll end with a quote from one who said it best;
We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.
—Donald E. Knuth