In an article not-so-cheerfully titled Seven Ways to Flunk a Job Interview, author Adriana Gardella shares an anecdote about a job-seeker whose Linked-In grammar earned her instant dismissal (as a candidate for said job).
Yes, grammar counts, even when you're engaging in a semi-social media venue.
Would you start a telephone interview with "yo, whassup?"
Of course not.
A little formality (capital letters to start a sentence, for example) is expected in any exchange related to a job.