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Pro Free and convenient large file sharing
Copy/paste videos, gifs, images, geolocation, documents, and even executable files. Drag over files for practically any time up to 2GB in size. Images can be compressed, too.
Pro Free of cost
The application is available free of cost.
Pro Privacy focused
In addition to the end-to-end encryption that provides privacy to communications, the app also includes self destructing timers for chats, and the ability to permanently delete messages, yours and theirs, from your device.
Pro Media messaging
Videos, gifs, images, geolocation, and documents.
Pro Channels for one way broadcasting
Broadcasting channels suitable for announcing things.
Pro Beautiful UI
Very customizable with a lot of clean looking options.
Pro Possible to send photos and videos without compression
Pro Supergroups of up to 200,000 people
In Supergroups, anyone along with admins can delete their messages from all sides. A message can be pinned to the top. All of the user's messages can be deleted by an administrator, and they can be banned and reported as well. Supergroups can have a public link (outside of Asia, due to high spam volumes), which is also a username.
Pro Two-step verification
You can set up an additional password to your account, which can be reset by your email. You don't have to use an email if you do not want to. If you lose access to your email or forget your password, you will be able to reset it via text message if your account is over seven days old and will receive it within a week (in case you recover anything before then).
Pro Save time with inline bots and link previews
Unfortunately, on the desktop videos don't play, but images are viewable without visiting popular websites like imgur and Instagram.
You can play YouTube videos and others within the app, so you don't need to embed code or anything.
With inline bots, such as @gif, @pic/bing, @vid/YouTube, etc, you can look things up without leaving the client or chat that you're in. Simply type the name and some keywords and wait for the results to load.
You can ping people to get their attention by @mentioning them.
Pro Saved messages
You can write messages to yourself. You can write some notes, add links, and see everything across all your devices.
Pro Desktop app handles HiDPI
The desktop app has a built in setting to scale the UI of the app up to 200% which allows for those with 4k monitors/screens to use the app without scaling issues.
Pro High quality calls
Uses the latest GPL audio codecs, such as Opus, an adaptative high-quality codec that supersedes AAC at any bitrate.
Pro Cloud chat keys are kept in different jurisdiction
Even though encryption used for cloud-based storage (e.g. Gmail, Facebook, iCloud) may not be as secure as end-to-end encryption or zero-knowledge storage, Telegram does the most that it can by holding keys to chats in different jurisdictions, requiring intergovernmental cooperation to obtain the keys. This is aside from whatever other security measures have been taken, of course.
Pro Open source client apps
Client apps are open source, making whole service more trustworthy.
Pro Multiplatform native client apps
There are native desktop, mobile and web client apps, that are optimized for speed and are light on memory usage.
Pro Customer support is within the app
Although it's volunteer based, but still pretty top-notch if available, which it tends to be.
Pro Unlimited storage
Unlimited number of files can be stored forever on the cloud. You can upload files up to 1.5 GB each.
Pro Group Voice Chat
This Discord-like feature adds to the interactivity of the application.
Pro Voice and video messages
You can post voice and video messages if you are too lazy to write text messages.
Pro Pinned Messages
You can pin messages to the top of groups or supergroups so members will see them when opening the group.
Pro Self-destruct messages
Messages can be set on a timer. They will disappear after the timer expires.
Pro Banned in China
Pro Popular in the USA
On SimilarWeb you can see that Telegram is on the top 15 among all communication apps.
Con Not fully open source
Telegram has an open API and protocol free for everyone, but the backend software is not open source. Therefore 100% privacy can not be evaluated.
Con Can only be used with a phone number
It is not possible to activate and use Telegram by mail etc. You need a phone number.
Con Only "Secret Chat" messages are encrypted end to end
The only way to get end-to-end encryption (client-client) using Telegram is to use the Secret Chat function, anything through the service is server-client encrypted. Also secret chat is only implemented in the mobile apps, meaning there is no way to encrypt messages end-to-end when using the desktop client or the web browser client.
Con "Secret Chat" feature is not available for desktop client
Secret chats with end to end encryption are only available on mobile apps.
Con No source code sintax highlighting
Telegram does not have source code syntax highlighting, making it difficult to share code snippets.
Con Does not work for visually impaired
Doesn't work for visually impaired.
Con Does not support spherical 360° photos
Spherical 360° photos becoming more popular these days, but Telegram does not support such format.
Con Does not support text messaging
You can NOT make it your default text messaging App.