In South Africa Twitter is huge. In Germany, Facebook is. Instagram is big in the UK. Now take the possibly not unusual scenario in which a South African lives in London and has family in Germany.
Granpappy in Berlin would search for his grandson in London from Facebook but finds him on Instagram. He'd add the grandson as a friend and see the grandson's Instagram photos in his Facebook feed.
The South African guy in London searches Instagram for his nephew in South Africa, and finds him on Twitter. And sees that Twitter account's posts in Instagram.
If someone on Facebook finds me on Instagram, Instagram could notify me that Mr. X is searching for a profile that matches mine, and ask if I want to allow access to my stuff. If I decline he can't. Federation, with user control and consent.
I think it could work. And even if it was useful to a tiny monitory at least there's real choice. Inclusive of all.