It is completely redundant to say "You can tweet us on Twitter at ...". Well that's nice, exactly where else could I tweet you? Either people know what Twitter is and can tweet you, or they don't know what Twitter is and now you're just confusing them. Depending on which of these people you care about more you should say one of the following:
- "You can tweet us at ..."
- OR "You can contact us via Twitter at ..."
You see? This isn't confusing or redundant and the second one has the advantage of letting people know that Twitter is a communication system rather than a "tweeting system", whatever the hell that is.
Think about how silly this redundancy is in terms of other communication media, it's exactly like saying:
- "You can email us using you email program at ..."
- "You can call us with a telephone at ..."
- "You can write us a letter and mail it to us through the postal system (or a private courier is you desire) at this address..."
- "I spoke to Bob Smith today in our Toronto studio via modulated air vibrations."
There are words like speak, write, call, email, tweet that tell us what the communication medium is. If you are worried that not that many people know how to use said medium you can just say "You can also contact through UnfathomableNewTechnology at ....".
Is that clear? Or do I need to come down and tell you in person via translating my body to your location in physical space?