I've heard a lot in the past few years about how online we're competing for attention and that people will only use x sites regularly. I'm coming to believe that view is slightly wrong.
We're not competing for attention but for memory. People do and will use many sites, but they just forget to do so on a regular basis.
I hear that all the time with DuckDuckGo. It's either part of your routine or it isn't. That's why I think the check-in effect is so compelling.
There are a couple others I've been thinking about as well to overcome this forgetfulness issue. You can think of them as addressing the problem by combatting the out-of-site, out-of-mind mentality.
First, there is presence -- simply being seen on a regular basis. This is pretty straightforward. Many services achieve it with an email or push notification or update. Others (like us) try to get you to set us as part of your browsing experience, e.g. via the search bar or an add-on.
If you're seen at just the right time and with the right periodicity you can be a site users visit without being one of those main x sites. I put BillGuard in this category. They send me an email monthly and I pay attention to it and almost always click through.
Second, and more subtly there is a being a starting point. It used to be people were vying to be your start page or browser. And people of course still do that and it is still valuable. However, the start page seems to have fragmented in the past few years. With mobile, you can have many starting places because you have many apps on your home screens. And the browsers have introduced more features (including native start pages) that make it easier to view a multitude of sites at once in a similar fashion.
I think the key to getting on one of these screens or bookmarks is to have people think of you as a starting point for something. This concept overlaps but is independent from the check-in effect. For example, you may or may not ever want to check-in to a photo editing app, but when you want to photo-edit you can think of that app as a starting point for that activity. It's then the startup's job to meld that association.
With social apps there is a sub-concept related to audience. That is, a social app can be a starting point if they are the gateway to a unique audience. I now use Twitter, Facebook, Foursquare, Posterous and Path all regularly. I do not syndicate everything across them (and actually not much of anything) because they each have different audiences and therefore I find myself starting a lot at each of them.
Path is an especially interesting case since they seem to be trying (via their UI) to be the new mobile social start page. The idea is you have your closest friends there and share everything with them, and then you can broadcast some of that stuff to Twitter, Facebook or Foursquare when you want to. This setup doesn't really work for me though because the audiences are so different.