Maybe it is being used that way, but I don't think it is a valid measure. I've had to join a few Telegram channels to participate in Airdrops. Once joined, I never look at them. And I knew nothing about the tokens before joining their channel, and probably won't learn anything about them before playing with the tokens, if they ever come.

So full channels on Telegram are probably a great indicator of how much it is being used to give stuff away. And there is no reason to connect that to the likelihood of the project succeeding.

But Telegram is also a really well designed service that handles large numbers flawlessly. There's a thread that I've posted to about why ICOs use Telegram here:
https://blockchainforums.info/topic/392/is-telegram-useful-to-icos