Besides the abstract "engagement" to shore up retention, more friends increases the ad surface without requiring an increase in users. An advertiser can target to more users, some of whom they might not normally be able to identify as interested.<p>For example, I am sometimes served ads for a breathing apparatus, for people who have things like sleep apnea. I have never seen an ad for such a device anywhere online. I have a FB friend who has a breathing issue and uses a device like this.<p>It seems like I would be a good target for an ad like this, because I have a Friend who might be interested in it, and I might point it out to her. It allows Facebook to charge more for ads, and makes advertisers more interested in advertising.