I admit that this may seem a strange question. One is a microblogging platform that allows you read and write messages of at most 140 characters. The other is a 3D virtual world. They are, however, both communication tools, and I think there is a completely different reason why Twitter is so much more useful to me than Second Life is.
If I go to the Twitter web site and log in, I see this:
It is Twitter. It immediately shows me the main content: tweets. It also allows me to create content, that is to tweet.
By contrast, if I go to the Second Life web site and log in, I see this:
It is not Second Life. It is a complex web interface that gives me access to account administration tools and shows me lists of blog posts by Linden lab, comments from Second Life users, items for sale on Xstreet SL, and video tutorials. In the lower left corner it shows me the only really useful information, namely which of my friends are online. That is, the friends that I would have been able to chat with, had I been in Second Life and not on the web site, which does not allow you to read or write messages.
Imagine if the web interface of Second Life would instead show me this:
It would be Second Life. It would immediately show me the main content: the virtual world. It would also allow me to interact with the content, that is to move around, to chat with people, and even to create content. Do you think Second Life would have more users if it would run inside your web browser? I think so. Linden Lab is and has been focusing on improving the initial user experience in Second Life to improve the retention rate (i.e. the fraction of new users that continue to come back). I am not saying that this is not important, but I think that most of the potential users are lost long before they even get into the virtual world.
This is by no means a problem that is specific to Second Life. Today, asking users to install a piece of software on their computer will cause the majority of people to shy away before they have tried your product. Even just asking users to create an account will cause many to turn around and walk away. When it comes to social networks, the decisive factor is users. If your friends are not there, why should you? Imagine a virtual world that would run in your web browser and which you could sign into using OpenID, Twitter Connect, or Facebook Connect. Would your friends be there? Would you?