An interesting analysis from Wharton marketing professor David Bell, from an article published on the KnowledgeWharton website:

According to Bell, there are strategies that social network sites can use to avoid becoming tomorrow's abandoned property. One way to retain a site's aura is to limit membership. For example, Bell notes that when Diesel jeans faced the problem of losing marketing cachet by becoming too popular, the brand cut back on the number of outlets it would sell to. Facebook tries to limit itself to college students. Social networks seem to operate best when they strike a balance between heterogeneity, which provides large numbers of members, and selectivity, which keeps the hordes focused and engaged in the site, he says, adding that social networking sites also must keep pace with technology and provide new features -- for example, fast downloads. "To create stickiness you must have functional value and also community value. If either of those becomes diluted, you give people a reason to start looking elsewhere."

So communities, in order to stay "current" should consider:

• Focusing
• Staying up to date with the latest technologies

I would also think that each community has a natural lifecycle, similar to anything human, and that regardless of what happens they cannot stay "hot" forever. I would even think that it is a healthy process to actually disband a community every once in while, similar to how in a democracy an elected government does not stay in power more than a pre-defined period.
If the community members really want more, they can always re-create the community, and it will probably not have the exact same form as before. And if it was a dying community, then disolution is probably a good and natural thing, offering members an opportunity to go and look for the next one if they had not thought of doing so already...