Totally agree on Facebook, but Twitter is much more valuable than the ephemeral tweets.
I think the difference between the two comes from the nature of user relationships. While on Facebook it had to be reciprocal (following was added later and it's not used much), on Twitter the user relationships can be one way. That makes all the difference. This is why on Facebook the user relationships are just replicas of real-life relationships, which was great for user acquisition but it's also a huge limitation that Facebook can't get past. On Twitter on the other hand anyone can follow anyone and while user acquisition is more difficult, it opens up the world and gives a lot of opportunity for new user relationships. Because of this Twitter is the best social network we have so far.
I think the difference between the two comes from the nature of user relationships. While on Facebook it had to be reciprocal (following was added later and it's not used much), on Twitter the user relationships can be one way. That makes all the difference. This is why on Facebook the user relationships are just replicas of real-life relationships, which was great for user acquisition but it's also a huge limitation that Facebook can't get past. On Twitter on the other hand anyone can follow anyone and while user acquisition is more difficult, it opens up the world and gives a lot of opportunity for new user relationships. Because of this Twitter is the best social network we have so far.