nofollow simply means search engines don't follow the link -- it has no negative impact on a site, but it doesn't add to it either like a normal (aka follow) link does. Some halfwits call normal links "dofollow" when there's no such thing, never has been and calling it that just encourages people to do stupid things (like say "dofollow" in their REL and META -- which is gibberish).
Some people get their panties in a knot over sites making links nofollow, and that's generally just more ignorant nonsense; any link is good. Follow links are BETTER, but any link is a chance for someone to visit, and if it's from a site like Facebook or G+ those nofollow links may in fact generate MORE traffic than search engines can depending on what people are searching for. It all depends on who that nofollow link is FROM. Many times the people sharing your links on social media are doing so with other people interested in your topic -- meaning you are more likely to get conversions from people clicking on those nofollow links than you ever would from search engine traffic.
But negative impact? That's just grade A farm fresh manure.