From time-to-time I'll post ideas. I may decide to pursue them, or perhaps not... I think it's always better to throw an idea around regardless, since success is often found in the execution (not the idea).
Idea 1: Social product review
This would be a service based around social product reviews. Essentially, users would register for the service and create reviews for products. Other users would be able to discuss the product, write their own review, or rate it's popularity. The user who posts the first review will receive a share of revenue generate from their review page. Other non-registered users would be able to browse what products are popular, what products are recommended, and what the overall rating of a product is.
Sociality begins when users connect with other's based on various interests. These socially connected users would be able to see their friends reviews, product recommendations, and perhaps items for sale.
Revenue would be generated by highly targeted ads linking to Amazon and eBay listings, perhaps eventually allowing users to upload their own listings.
Idea 2: Personal inventory
This would allow users to quickly create a personal inventory of items they have (perhaps items they don't want). Optionally, users would be able to tag and rate the items (or even review them); all of this information would go into a database indefinitely. Users could then connect with other users who have items in common to see what they might be interested in (as a sort of recommendation engine). These users could connect and discuss their interests and build a primitive network, perhaps buying, selling, or trading their items.
Other users could search through the database to find users with a specific item. They could then connect with the user and see if that user is interested in selling or trading the item.
Idea 3: E-mail based social network (objective unknown)
E-mail is the most primitive form of social networking. Many people have built at least a basic list of connections in their address book -- if they haven't, they generally know (or can find) the e-mail address of friends.
This service would allow people to upload their address book. As other users upload their address book, connections would be made. If user A has user B in their address book, and user B has user A in their address book, then a connection is made. If the A<-->B connection doesn't exist, it's left "open"; user B is made aware that user A is connected with them, allowing them to decide whether or not they want to "add" this person.
This would build a real-life social network with the people you know. The objective could be wide-ranging, from commerce to life tracking. Users could extend out by several degrees when looking for an item for sale or requesting some sort of information.
The virility of such a service could be pretty high, as users want to see who is or isn't connected with them.