Twitter has been trying to sell advertisers on the 500M people it says arenn't signed up for the service but who nevertheless, read tweets to keep up to date for certain purposes.
The problem though is that even if this is true, if the advertisers can't quantify the value added by these non-subscribers, they have no way of knowing what to pay twitter to advertise. Of course that's probably bullshit since they can still get a rough idea of how many people they're reaching through twitter.
On the subscriber front, they don't seem to be growing their base as fast as they need to.
It will be interesting to see how this shakes out. Personally, I'd wait a few days and see what the market thinks before buying, but that's me.