These two paragraphs in particular sum up my suspicions that page tabs are probably going to be discontinued at some future date:

In contrast, page tab applications are rarely discovered unless a page directs an ad campaign to the tab. These apps then struggle with re-engagement because users rarely return to fan pages after clicking Like. Another issue is that page tabs aren’t available on mobile devices, whereas many Open Graph apps are. For example, Yahoo has a mobile web app version of its social news reader, which Facebook says was getting 1.4 million visits a day in February.

Although Static HTML and other page tab apps still have millions of users today, we can only foresee their numbers continuing to fall. As mobile usage increases, the potential audience for these apps shrinks. There is also the growing trend of pages running Sponsored Stories and page-post ads, which unlike traditional Facebook ads, cannot lead directly to a tab application. We can imagine Facebook completely eliminating its traditional ad format within the next year or two. It could also do away with third-party page tab applications completely in favor of the Timeline activity box display it uses for personal profiles. Either way, it’s clear that Facebook believes in Open Graph as the future of its platform and developers will need to respond accordingly to maintain relevance moving forward.