Can an employer mandate posting company posts on personal social media formats?

Typically, short of legally actionable discrimination or having an employment/union cagreement to the contrary, an employer can set the conditions of work much as it sees fit. This is known as "at will" employment. Accordingly, your company can mandate that its employees post company blogs on their social media (or pretty much anything else for that matter). For your part, you aree free to quit as you choose. Either that or comply with company policy. If you don't you can be fired. In fact, you could be terminated for any reason or no reason at all, with or without notice.

Yes, it is legal. Employment in the U.S is "employment at will"; among other things, this means that the employers may set terms, conditions, requirements, etc. on employment. Those can inlcude an obligation to post company blogs on employee social media. Employees who do not comply may be terminated; employees who do not wish to comply should seek other employment (since part of "employment at will" is the notion that not only can the company fire you at will, if it doesn't like you or want you as an employer, but you can walk away from the company at will if you don't want to work there; the expectation is that if you find terms or conditions of your job unacceptable, you will find another job).