I guess what im wondering is more about the scalability of it. I know that Mongodb could essentially be used as the only datastore however it apparently has some issues currently with scalability.
So, your app is unlikely to be as popular as foursquare, for instance, at least right now, and they seem fine and happy with MongoDB. You might recall the issues they had with it a while back, but those have all been fixed as far as I'm aware. 10gen is really quite impressive when it comes to support, etc.
But, the real question is, why do you think a SQL database will not be scalable to begin with? I'm going to say that well over 75% of the top sites, use a SQL database.
Mongo 1.8 fixed a lot of the scalability issues. +1 to Mongo as an alternative to SQL. Redis is great for a simple app but I wouldn't want to scale it to handle everything for a large site.
But, the real question is, why do you think a SQL database will not be scalable to begin with? I'm going to say that well over 75% of the top sites, use a SQL database.