On Facebook you're generally connected to people you know in real life. So if a new person joins Facebook and connects with you, the FB recommendations are like

On Facebook you're generally connected to people you know in real life. So if a new person joins Facebook and connects with you, the FB recommendations are likely people the new person also knows. You frequently follow people on Twitter you only know from Twitter. Even if they follow you back, your friend just signing up may not know who the hell they are. So recommendations are more difficult.