The way I interpret "dead meme" is,that if its rarely being used by the majority of people and no longer as popular compared to other memes, then its is to be considered dead or dying, yes there are memes that get a start from here, but honestly most of the memes that blow up, come from reddit, then go through a cycle, of being posted on other social media platforms. Here is the cycle for example: Reddit: (or sometimes Imgflip): popularity begins.
Twitter: the meme is still relevant
Instagram: the meme is losing popularity, and slowly dying.
Facebook: the meme is dead, and is being shared among people who aren't up to date with meme culture.
imgflip: the meme is now being used as a amongst it's users, and will not make it out of its websites bounds, thus making this the afterlife.