When applying to employers who do not have specific positions listed, obviously writing cover letters and finagling resumes for companies that might not have matching positions available can be a huge waste of time, so at least in this case it seems like initial contact via less formal methods seems like it would always make sense... I would like anyone to weigh in for reasons that make that not a fair assumption.
BUT, for positions listed, especially when applying heavily, there may be questions a job seeker would like to ask before applying, so as to reduce time spent applying for work that may not be a fit. Is it appropriate to make contact informally before a formal application? I would wonder if employers might get overwhelmed if many applicants do this. I wouldn't want to take myself out of the running right at the outset by annoying an employer with questions about a job opportunity.