There is a lot of talk about Google+ being more of a threat to Twitter than Facebook. While I don't deny that is true, I think Twitter still allows for great social discovery. When people retweet or reply to tweets it engages people from the outside.
I joined Twitter late compared to most of the people I interact with. I did not see the value in it at first, but gradually began to understand how important it is in keeping up with the open source community. I signed up and started following those same people whose blogs I read. As those people tweeted I gradually began to follow more and more people that interested me. The re-tweet is a great for discovering new blogs or projects that interest me. A re-tweet is an awesome way for someone to point out something they think is interesting. The re-tweet takes only seconds as well. Twitter conversations are harder to follow, but it is a conversation fragment is usually enough to decide it the conversation is interesting enough to find the main thread. The Twitter website does try and track what tweet was in reply to another tweet, but it is definitely lacking.
I have yet to find this same sort of social discovery on Facebook. I really don't use Facebook that much because I feel that I am interacting with the same people I always interact with. I do see new faces that comment on posts of people I follow. However, I want to see people I follow commenting on posts of people I don't follow. Even better, I want people I follow to highlight a good article, blog post or conversation. I feel that when people comment on something, they are less likely to then re-tweet it. Most of the re-tweets I see have a short opinion in front of them, such as "Amazing read! RT ...".
All the cool new bells and whistles of Google+ are great. However, I think Twitter will still remain important among the tech-savvy crowd until Facebook or Google+ provide that a better method of social discovery.