This is something I've actually heard a lot — which makes sense, since I'm someone who works (no quotation marks!) from home and who is, for a period of time each day, pretty active on Facebook and Twitter while doing said work. As such, there are two BIG things I feel like people should know.
First: I actually AM doing work
Like all the Empire sites, Offbeat Mama has Facebook, Twitter, and Pinterest accounts. As such, I regularly sign into all three sites to moderate comments, reply to tweets, or pin snaps of kid birthday party inspiration. I also have a personal account on these three sites, and so when I'm finished with, say, reading through the Facebook comments (since many people will comment on Facebook OR Offbeat Mama, but not both), I'll switch over to my account and upload a few photos of my son or link to the song I'm listening to. Inevitably, I'll click over to a few friends' walls, leave them notes or messages, and then probably sign out. So while it shows up in your stream that I said something to someone, posted a photo, or was spending thirty minutes listening to Beatles songs, what you DON'T see is that I spent time checking out the page for my job first.
Second: your office job has something my from-home job doesn't: co-workers
Think about your out-of-home job: you have a lot of stuff that a from-home worker probably doesn't have. Stuff like regular work hours, health insurance (as Ariel likes to say: *sad trombone*), and built-in adults you can talk to about your job or whatever else you want to talk about. Granted, I make my own hours and love this (even when those hours end up being 8pm-2am and I'm exhausted the next day when my son wakes up), but the co-workers being nearby would be pretty rad (not to mention, um, health insurance).
The closest I get to face-to-face contact with any Empire staffers is during the video editorial meetings we have every Friday. Those are fun, and entertaining discussion goes down, but they're thirty minutes and by necessity are filled with a lot of editorial content in between the fun. So if I want to show Megan this fucking adorable photo of a kid in a Darth Vader princess costume, I usually send a link in Google Chat or in an email. She still sees it and reacts, but neither of us get the instant gratification of "OH MY GOD THAT'S SO CUTE!" that would probably turn into an hour-long Star Wars discussion while Cat shakes her Trekkie head at us and Ariel schemes ways to turn the Darth Vader princess costume into a Darth Vader Lolita ensemble for herself.
My point is even if you're not fortunate enough to have co-workers who would be into a pink-bedecked Darth Vader, you at least have them in your immediate vicinity. If you did want someone to join you for lunch, you can ask someone nearby. If you read a really funny joke and just HAVE to get it out, there's probably a few pairs of ears that would be receptive. Until we started the video chats I didn't even know what Megan or Cat sounded like, let alone what Catherine LOOKED like.
My point, or at least the one I thought I was trying to make, is that when I'm home working, social media usage is also my human interaction. Granted, I'm not home all day long in the same spot, typing away, so I do see other people (even adults!) in my daily activities. But there's still something different about hanging out with people while you're working that actually get what you're doing, instead of having to fumble for ways to explain that you work from home and really are working. Really. Most of the time.