What happens after a meme loses popularity?
Eventually, people start to see a popular meme so often it gets boring, and the meme's popularity starts to decline. Once a meme falls out of the spotlight and fades away from constant usage, one of two things happen to it.
Outcome one - the meme stops being used almost completely, and is considered to be "dead." The only remaining uses of it vary completely from its original state, and those uses are widely regarded as unfunny and lame. "Harambe" is a great example of this. It rapidly rose in pouplarity, and as quickly as it came it disappeared.
Outcome two - the meme loses a lot of its popularity, but still "lives on" and circulates around smaller demographics, such as online communities, or simply isn't used as much as previously. The "Lenny Face," while not as mainstream as it once was, is still widely used in a variety of places on the internet and a lot of users will still recognize it.
However, memes aren't always one-hit wonders. Some will rise in popularity again after a resurgence of new takes on the format, such as "Doge" from the early 2010s era of memes. The meme has been considered "dead" for the past several years, as with most memes from that time frame, but has recently seen an increase in usage over the past year and a half with people creating a sort of lore based around the character and other variations of it. Other "dead memes" such as "Troll Face" and many others never gain traction again, and are used solely and rarely in an ironic or a "so unfunny it's funny" sense.