Being both an artist myself and a lover of music, this is a subject that I can most definitely see from both sides of the looking glass, but I still have a single one sided opinion on it.
If we break down the original intentions of the creators of Facebook, Twitter, Instagram, even the newly-hyped Snapchat, they'll all share a common factor: they were all designed for people to share what they're doing and things they've done with anyone that's interested in a fun way. You know someone, follow them, talk, share pics etc and boom, job done. The possibility that when you think you're speaking to your brother on Facebook you're actually speaking to a complete stranger pretending isn't something you're likely to consider, what would be the point? And to be honest I don't see why this should be any different with popular musicians.
Okay, the first thing that probably springs to mind is: 'They're busy! They don't have time to reply to fans and post up pictures!' And while that may be true, I don't see how that justifies having someone else pretending to be them. If you don't have time to post up pics and tweet what what you've had for breakfast that day then don't, that's what non musicians do, and it also means that when you do actually put up something that genuinely came from you, your fans will be a getting a genuine feel of who you are.
I understand online visibility is very important, and I don't see anything wrong with getting someone on your team to post in your stead if its just a notification of a gig or record release, but in my opinion the line is crossed when you have someone posting things like 'loving blah blah blah on stage right now' or even worse, actually replying to fans acting as if its them. I had a fan send me a lovely Facebook message the other saying how much he enjoyed my music etc but ended it with, "I'm not sure why I'm writing this as the reply will probably just be someone from your label" and it took me about three additional messages just to convince him it was in fact me that was thanking him for his kind words. Now I'm literally a new artist still building a fan base, so it was a little heartbreaking to see that fans are expecting an equivocal response even from someone at my level.
Going totally off tangent this is probably the first time I've ever used the word equivocal in anything and I'm very chuffed the word popped into my head. Till next time, peeps!