"Thumbs Up If You…", also known as Like If You Agree, is an expression used to preface pleas for users to upvote a comment on the video-sharing site YouTube.
On August 23rd, 2007, YouTube released an update to its comments section interface that allowed users to "thumbs up" or "thumbs down" comments. On March 25th, 2010, the site rolled out a new "Highest Rated Comments" section, which prominently displayed the highest voted comments in a separate area on the video page. Soon after, comments containing upvote pleas began appearing on the site.
On September 9th, 2010, "thumbs up if" comments were mentioned in an article on the Internet news blog Urlesque about as "proof that humanity is doomed." On March 14th, 2011, YouTuber ADoseofBuckley uploaded a video titled "YouTube Comments," in which he laments the thumbs up pleas in comment sections (shown below, left).
On June 1st, the user interface blog UX Movement published a critique of "thumbs up" and "like" buttons, arguing that the upvote comment games stifle meaningful conversation. On May 7th, 2012, the pop culture blog Seoul Beats published an article criticizing cliche YouTube comments, listing "thumbs up if" among the examples. On June 27th, the Internet news blog UpRoxx published a slideshow highlighting screenshots of thumbs up request comments. On March 18th, 2013, YouTuber undertakerfreak1127 uploaded a public service announcement video asking for an end to "thumbs up if" comments.
"Thumbs up if you agree" is used as a simple appeal for the viewer to upvote a comment if they agree with its premise.
"Thumbs up if X sent you here" asks visitors from specific websites to provide up votes to a comment.
"Thumbs up if you were the X viewer" comments are often used to joke about the frozen view counts displayed for new YouTube videos.
If You're Watching in…
YouTube commenters often post a variation of the phrasal template "Thumbs up if you're watching in X," where "X" is represented by a specified year. The practice has existed since as early as July 2011, when a question was submitted to Yahoo Answers asking why commenters were posting future years in the phrasal template. On August 25th, Urban Dictionary user themanclaw submitted an entry for "Thumbs up if you're listening to this in 2011," defining it as a popular YouTube comment for music tracks uploaded to the site. On February 11th, 2012, "Thumbs up if you're watching this in 2011," was added to the "Memes: YouTube" page on TV Tropes. On August 27th, Urban Dictionary user Chowder183 submitted an entry for the practice titled "Thumbs up if you're watching this in…" On March 19th, 2013, YouTuber undertakerfreak1127 uploaded a video criticizing the commenting practice (shown below).
Commenters subsequently began posting a copypasta message mocking the phrasal template, asking viewers to up vote their comment if viewing the video over the course of the next 100 years (shown below).
 UX Movement – How the thumbs up like button is dumbing users down