LinkedIn helpfully provides readymade comments on the updates that your connections post there. If your colleague Luisa has a new job, you can just click on a button below her update to post an immediate Congrats Luisa.
That should be Congrats, Luisa, however. To get technical, this is a vocative construction that traditionally requires a comma before the name of the person being addressed. Similarly, it should be Thanks, Denis and Hello, Yolanda.
In modern professional correspondence by e-mail (or LinkedIn posting), the vocative comma is rarely seen. I suppose I can live with that in the era of Hi Neil as a salutation (and I am one who drops the (non-vocative) comma after Dear Sir or Madam and Yours truly – even in a formal letter).
There are times when the vocative comma is essential, however: I know Ali and I know, Ali mean different things (which should require no explanation).
On a related point, there is some ambiguity in placing the sign-off Thanks, John all on one line; only when the name appears below the Thanks is it clear that it means With thanks from John rather than Thank you to you, John (although actual confusion may not arise in practice).