There is no big mystery about this. Bump means up! In a facebook discussion, threads move down, down, down and soon after it gets lost. So if you are the person who started the thread, you may just want to bump it up so that it gets seen again.
This is especially important for people who are selling stuff in the new Facebook For Sale groups. If you are selling something and it still has not been sold after a while, it goes down, down, down. So you type in “Bump” to move your item for sale up again so that it gets the chance to be seen. However, do take note that not many Facebook groups, forums or For Sale groups likes this action and may even ban it especially if the seller or commenter keeps on bumping his posts up so if you are thinking about doing it, do it with discretion only occasionally.
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