What I imagine is that living people will be able to posthumously create Facebook pages for their ancestors, and create relationships between them (and their living relatives on Facebook). It would certainly popularise the whole area of genealogy, and encourage people to trace their roots - and generally enrich the information/relationships about previously unknown relatives.
Would it reveal the truth about how many people really are descended from William the Conqueror or Abraham Lincoln? Probably not, but it would be interesting which patterns do emerge.
As for the current leaders in the genealogy field, such as Ancestry.com, I don't think they'll need to worry just yet. Maybe in a few years they'd have to consider whether to raise their game or stick to their business model, but for now it's business as usual.