I think it is somewhat clear at this point that it is net bad. While I cannot comment on the claims made by some FB employees rising to defend their own moral choices I can say that I left FB when the consequences of my work on personal privacy could no longer be ignored. Luckily for me (I guess...) this was before it became clear that there was also the long-term destructive effect that FB has on democracy and civil society to deal with, but I think you should know that working there is always going to involve compromising your morals and ethics. You and your co-workers will be surrounded by a lot of motivational posters and routinely be told how your work is serving some nebulous positive benefit, but it will be a lie.
I have worked for a lot of start-ups, including several that grew to become significant giants in their segment of the internet. Facebook has been the only one I worked at where there was a group of employees whose job was to create posters to hang up in all of the offices telling everyone else how important and worthwhile it was to work at FB -- looking back I think this level of internal propaganda should have been a warning sign.