Twitter will grow best as the service we know on it's own or with an owner who does not have a need to morph the product into their existing services. However there are many benefits by integrating... and may downsides.
One can see how a search provider would benefit them (and themselves) by showing updates in results better and posting back to results.
On the other hand facebook brings a huge community but also the risk of turning a tool that people may use for one purpose into a part of a profile where it doesn't fit (groups & uses).
Selling and taking the cash is very different than developing down the road, or even developing revenue. They all can support each other and conflict depending on the play.