You should look at the total market for the parent product, and come up with a reasonable percentage of those users who will buy your add-on. For example, if there are 5 million iPhone users, and 5% buy a screen protector, then you're looking at a total available market of 250,000.

From here, you'll have to figure out what percentage of the market you can reach, etc.

like twitter? i generally dislike those companies.

no idea really, look for some successful companies that have built upon other platforms... facebook games probably a good start.