- Did you know you could follow us on our Twitter?
- Did you know you can follow us on our Twitter?
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Both of these are corrrect, and in practice there is no significant difference in meaning for a US Native speaker between "Did you know that you could X" and "Did you know that you can X".
As @TypeIA points out "on Twitter" is more common than "on our* Twitter". But I would not call either incorrect. An alternative would be "on our Twitter feed" or "on our Twitter channel." A feed or channel is associated with a particular user account as all of Twitter is not. But simply "on Twitter" is still more common, in my experience.
While the statement by @Jeff Morrow about the difference between "could" and "can" in this construction is probably technically correct, I think that in practice this distinction is rarely if ever intended, and a learner would be best to treat these as of identical meaning.