Lots of good insight in this short article: http://bokardo.com/archives/everyday-app/
It's interesting that across the world, the number of sites that we use on a daily basis is relatively small and consistent. I can attest to this, as I fit pretty much in the 7-8 range.
Quite often when designing a site for use on the public Internet, we obsess over all of the little details that take up the majority of the development cycle. This evidence suggests that we should do otherwise.
Think about the relationships we have with brick & mortar stores - we probably also have just a few that we frequent on a regular basis - grocery store, gas station, dry cleaners, bank, etc. All of these stores are designed with efficiency in mind. They want you - the customer - to get in, get what you need, and then get out as quickly as possible: grocery stores now have self checkouts; banks have drive-throughs and ATMs; gas stations let you pay at the pump, and so on.
Stores that we only visit on a monthly or less frequent basis have a different and more complex challenge - actually getting us to go there and then keeping us coming back. Their marketing challenge is far greater than that of the grocery store, as they know that they are not a commodity site, and we - the customer - have more of a choice. These types of stores include furniture stores, car dealers, specialty stores, and so on.
The experience with one of these stores typically includes some sort of interaction with their sales staff. They don't focus on efficiency as much, as they prefer a more guided sale, which often includes attempts at up-selling us to a more expensive product or option.
Thus, when we're building a site that is not classified as an "every day" site, - which may be more frequently than we had originally thought - we should take care to do the same. Focus on ease of use and simplicity. Add in as much guidance and make it easy to contact someone, should the user deem that necessary. We have to assume that the visitors of our site are not frequent visitors, and we only have a few seconds to entice them to become a customer. Thus, the entire experience should cater to this fact.
One final point - I totally agree with one of the comments regarding Linked In. I have also signed up, connected to most of the people that I know, and have the same "now what" feeling. Please let me know if I'm missing something!