The 15 character count can be restrictive, hence, for .com's 13 letters or more, adding (1) com, (2) _com, or (3) dotcom simply isn't an option.
I like the descriptiveness of *dotcom, however, this only works for .com's 9 letters or less.
Does *Com work best when the prefix is a company, such as AppleCom working best because Apple is a company that also has their .com? Or does this remind, or confuse the brand, to something like WorldCom
And at what point does one consider adding a prefix to their desired twitter handle rather than adding a suffix?