This highly depends on the platform. Some offer this feature, some don't.

Facebook Profile: it's not possible at all. The only way is to actually share your Facebook login credentials.

Facebook Page: you can invite others to be in user roles to be able to do different things (marketing, posting, analysing, moderating).

Twitter: with TweetDeck by Twitter you can invite others to be able to post and reply under your name without having to give them access to your Twitter account password.

Instagram: not possible to do aside from sharing the password. However, what you can do is use a service like Later.com to let others upload, write and schedule the uploads. But you will still have to do them manually on your phone.

Snapchat: also not possible.

This list is of course - as of this writing - 2017-08-16. It might change as the platforms develop further.

Overall, Social Media administration is something that involves a lot of trust - or hard work on your part. And you have to decide which part it should play in your business. Do you want to be a business online or a personality? Which of these platforms is important for you? And on which can you get away by letting others post for you?

Maybe it's not so much the posting itself but rather the content curation and creation that needs to be outsourced? Posting itself is the easy part in many cases.

If you need more ideas on how to do this. Which kind of automation I would suggest for the different platforms. Let's talk and we can work on a concept together.

If you are friends with this person on Facebook you can go into your Page Settings and edit Page Roles. Assign them the role you wish, depending on what you want them to be able to do. They will then have access to the Page. In your case I would assume you would assign whoever as an Editor. If you're on your desktop and doing this, Facebook will give you a layout of who can do what in different Page Roles -- you will see link at top of page when you hit Page Roles under Settings.

Regarding the delegation and automation part of your question. It is possible for you to use a scheduler like Smarterqueue or Hootsuite, which ever suits your need, then get your FB Virtual assistant to upload all your articles, and posts etc (I use Smarterqueue!).

You assistant can also determine what articles, and posts did well and which didn’t, and amend accordingly, from the scheduler stats.

This would take care of the automation part, but wouldn’t you still want to engage with the people that respond to your posts? Particularly if you’re trying to build up a following, to sell your book and get clients. Maybe consider developing a social media strategy, if you haven't already, so you and you virtual assistant know what the aim is, otherwise you may end up just wasted your time, energy and money.