I've found myself not utilizing Twitter's services very frequently over the past few weeks. Although my schedule has been disrupted by travel and odd work-hours, the most telling sign for Twitter's potential decline is my lack of caring. No longer do I feel the urgency to view the constant glow of the Twitter stream - stemming either directly from Twitter.com or from any of the Twitter-enabled applications.
Although I am just one person - whose personal habits probably have nothing to do with larger trends - it has been documented that Twitter's traffic over the past few months is stagnating. I have encountered a good deal of anecdotal evidence of similar usage trends amongst friends and family.
It would be a revolutionary leap in Internet-enabled trend cycles if Twitter has indeed already crested. Only a few months after culturally peaking as the darling of Oprah, Twitter now faces a potential ceiling. Although this slowdown may be quite temporary - as was the case for other hits such as Facebook - it may also signal a deeper and more fundamental lacking of the Twitter value-proposition.