I am wondering: how feasible it would be to start developing a social networking website entirely based on silverlight;
This has been fairly discussed over the years in favor of HTML.
Has something changed with silverlight improvements over the years?
-- active users
-- technology used, MVVM + MEF (possibility of lags, server memory overflow...)
--- WCF Ria Services & EF
What are your thoughts on this subject?
Here is a non-technical angle to your question.
While Silverlight is an extremely developer-friendly platform for building rich internet applications, I would not consider it an option for building a public website which aims to achieve large penetration. IMO, social networking website is one of the worst use cases for Silverlight I could think of.
The value of a social network is directly related to the number of members in that network, and there are a significant number of users who will not, or can not install Silverlight to use your site. It would also make mobile scenarios difficult to implement, and without mobile support your site has already lost the competition.
Furthermore, in spite of the fact that I do work with Silverlight a lot, I would not necessarily choose it for any web project. Microsoft has recently been wavering in its commitment to Silverlight as a web (as opposed to WP7) platform, although the news of its demise turned out to be highly exaggerated. The development community at large is looking for HTML5 to provide the necessary tools to start creating standards-compliant, cross-browser rich web apps, and while the standard is not mature yet, IMO it's still a better bet for creating future-proof sites.
You won't be creating a Social Networking Website with only Silverlight. You'll be creating a Social Networking Application.
And yes, it is feasible. Seesmic Desktop is a Silverlight based application that allows you to access your Social Networks.
As for your concerns:
Silverlight runs entirely on the Client side so performance is going to depend entirely on the client hardware. On the upside, you won't have to worry about issues like the number of concurrent users or memory issues.
Silverlight has all the features needed to secure your application appropriately assuming you implement everything appropriately.
What you haven't mentioned is what Social Networking Services you're going to connect to. If you're going to create your own, you'll need some kind of server side services to support the operations in your application. Your questions seem more appropriate for those areas...but none of them depend on the use of Silverlight.
The secret sauce for a social network is the API -- and once you have that nailed, who cares what client tool is used to develop for it. I would expect a number of client flavours to be desirable -- not unlike the plethora of twitter clients (in html, silverlight, air)
Telerik has a facebook client built in silverlight you may want to look at.