It all depends on the specifics of the giveaway. If there is any "chance" involved in the prize, then you might have a lottery on your hands which is prohibited here: https://www.fdic.gov/regulations/laws/rules/1000-2200.html.
(State law could also have impact.)
If there won't be any "chance" in the prize, then it shouldn't be too big of a deal though typically EHL and member FDIC rules apply as well as possible bonus/IRS implications depending on the size of the gift. You should also be aware of FB contest rule as they have limitations on shares and tags.
I actually just published a (free) video on Facebook contest considerations that might give you a few things to think about, so I will PM you the link.
Adam Witmer, CRCM
All statements are my opinion, not those of my employer, and should not be taken as legal advice.www.compliancecohort.com