Is it common for startups to sacrifice their employees' social capital at the altar of "growth hacking"?
If it doesn't fit into the confines of something I would reasonably share, I don't share it. I've never gotten blowback for it.
On the other hand, at one place an email went out saying everyone was required to make a tweet or fb post about our new promotion. I immediately went to my manager and said there was no way that was happening by me, and as many of us in the company had overlapping social networks, it would be very transparent and spammy looking to outsiders. The language was changed to "highly suggested" and I was never bothered about it again, but it left a bad taste.
Gently asking for a like or upvote is one thing, but I'm not writing or posting anything on my personal accounts on behalf of the company.
If you work in marketing, you may advertise on your resume how many contacts you have in your social networking circles. I do not. I use them to keep up to date with family and friends. For me this was all very unexpected and abrupt. Please don't do this to your employees. I quit less than a month later and at that point I deleted every single mention of the company from my social network. I didn't do that because I disliked the company's product or even the people who worked there. I did it because I felt dirty for what I had been forced to do.
I think the important thing is that it is clear up front when taking a job whether or not your "private" social network will be controlled by the company. If I had known that they felt they had the right to control my posts, I would not have taken the job.