I don't think Facebook can make their messenger a memorable brand. The actual brand that people think about is Facebook. Messengers these days are a dime and a dozen, with Facebook's messenger being popular because it is "at hand", but that's due to people being active on Facebook. But it's not like Facebook has multiple products, their messenger being part of what Facebook is. Before Facebook my friends where using Yahoo! Messenger. And now it's pretty much dead.

"gchat" was actually Google Chat and was about voice and video chat. Now it was replaced by Google Hangouts. It's a pretty cool service, except that they have had two problems - their fuckup with Google+ and the popularity of Skype. Now Skype is a very memorable brand. And it has lots of problems, but people keep using it because it works for group video/audio calls and because it has good prices on calling phone numbers - like, Skype has a freaking monopoly on emigrants and their families or on companies working with remote consultants or employees.

In other words, I don't think the actual name matters that much, plus if you pick a weird word you might get more out of it. Googling is now a verb in the dictionary and tweeting should probably be as well.

I don't know about "the Watch"; it's more that they constantly push you to say things like "I'm using my iPhone" rather than "I'm using my phone", whereas with the "Apple" products, they seem to encourage you to just mentally commoditize them and say "I'm using my watch."

I could hazard a guess at the reason, too—everyone knows what a phone or a tablet (or a laptop, or a PC) is, and theirs don't break from the trend, so they need to differentiate their product in those lines using the name. Whereas, with the "watch" and the "pencil", they're making a new thing that attempts to redefine the category it's in, and so they want people to start using that name for the category as a whole. Specifically, they want people to think of any accessory-computer used for passive alerts and alarms and checking notifications as a "watch"; and a stylus used primarily for finesse drawing with extra sensory features to enable that as a "pencil." (The latter is also, I think, trying to avoid the current image of a "stylus" as effectively a prosthetic finger for those dumb devices that can't read your touches without it.)

Those are metaphors and metonyms. MS Word is not a word. MS Office is not an office. Facebook Messenger is literally a thing that carries messages. Apple Watch is literally a watch.