When they talk about the "user experience" they mean someone who is buying ads, not the person who is posting "Look what Hillary Trump said last night" every day. Think in terms of Facebook's customers.
Knowing who is talking to whom is an important part of Facebook's marketing. Look at how Facebook targets and consider item #19 in that article. It's not just about who you are, it's about who you know. Whether you think this is a good idea for Facebook or not, it is what they do.
User A and user B are friends in real life, use Whatsapp, and have Facebook accounts -- but they're not "friends" on Facebook (maybe they only use Facebook for work, or something like that). (Or maybe they don't have Facebook accounts, but Facebook has profiles on them gathered by "like" buttons, and has some way to deliver ads to at least one of them.) They communicate with each other using Whatsapp. This lets Facebook connect the two profiles, even though within Facebook alone, they are unconnected. The result: Now user A can see shopping ads for user B's upcoming birthday.
The advertiser has a good products experience.