Depends a lot on your current team. You can get through most of the current game with some decent lapis units like Edgar, Vivi, Shantotto, Cyan, etc + the main characters. If you've pulled one or more 5* high tier units, I'd just stockpile. I'd also recommend to not sweat using lapis towards upgrading unit/material/equipment slots. Units aren't bad for now, but wait until an EXP cactuar event rolls around and you'll find yourself wanting to stockpile lots.
If you're adverse to adding 300+ friends on FB (using your main FB account, for example) then adding friend slots is also a good investment.
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