Which are the features of the most popular social web sites?

Is it better to have one web app with tons of features or a few separate apps that tie together seamlessly? 

• While I know there are technical advantages to having apps separate, but interlinked, I'm mainly wondering from a business/marketing perspective.  I run a web app, which has one free and one premium plan, that has a multitude of features related to a CMS of sorts.  I'm looking to create a higher tier plan that offers two new big features un-related to the CMS.  The idea is that users can sign up for the higher tiered plan and get access to all three web apps (from one login, so it acts as one app in a sense) or if they prefer, they can just sign up for an individual site. My biggest concern is that I'm going to have an impossible time selling them on the features of the CMS and then all the features of the other two sites without sending them to other marketing pages.  The only example I can think of is 37 signals and I don't quite think I can surmise much from them since they're so big at this point.   Edit: I want to best leverage the momentum of the original site, but simultaneously don't want to throw tons of stuff at them as most customers will still only be interested in the CMS.  

• Answer:

  I guess it all depends on whether you want all these features working all the time for everyone, if you just want people to select the features they wish to use, it would probably be better to just have them activate the different parts separetely. You have to remember, not everyone wants everything. :)