Apparently if someone snagged your preferred Telegram username there is a way to snag it back.
(Telegram FAQ) Q: What do I do if my username is taken?
Telegram usernames are distributed on a first come — first serve basis.
We understand that certain usernames are part of an online identity for some of us. If your desired username is already taken, we will be happy to help you acquire it, provided that you have that same username on at least two of these services: Facebook, Twitter, Instagram.
Please contact our in-app Support if you have more questions (Settings — Ask a question). Notice that Telegram support is done by volunteers (so please be nice).
This policy should make it easier for celebrities, politicians, geeks, business gurus & brands to acquire their preferred username minus the trademark lawyers (although the latter would probably not work against Telegram).
Surprisingly other services like Vine, Google+, Flickr, Pinterest, LinkedIn, etcetera, are not cited by Telegram (although this may reflect a growing consensus that Facebook & Twitter “own” the digital identity space).
Note: I was fortunate enough to obtain @Darnell upon Telegram early on, although I also use the same username handle on Twitter & Instagram (no luck upon Facebook however).