These certainly do exist, e.g. salesforce.com are huge. However you touch on a point that hasn't yet been developed - you could innovate around actual service

These certainly do exist, e.g. salesforce.com are huge. However you touch on a point that hasn't yet been developed - you could innovate around actual service connection through a b2b social network. So exposing web services that could actually allow companies to place orders from suppliers or create projects they wish to go out to tender and the suppliers can bid on etc. I would think you're much better off focusing on a niche market, if you can prove one exists. If not then start generic and iterate, honing in on a target market. But be aware there are definitely competitors in this general area.