Being a brand advocate shows up in many community jobs. It even appears as one of Jeremiah’s 4 tenants.
The problem should be obvious. A community manager can’t spend much time saying nice things about a brand without sounding like a cheap shill.
If you sound like a cheap shill, no-one wants to know you. You can’t build strong relationships with members, nor do what you’re supposed to be doing; developing a strong sense of community.
You do need to mention what the brand is doing. You might mention genuine news such as the release of new products, upcoming events and exclusive information, but there is a line. That line is between being informative and trying to be overtly persuasive.
For example, "We’ve just released Widget X" –vs- "Buy Widget X now!". One is providing information the other is shilling.
These are extremes. There is plenty of gray area in the middle. There is a lot of subtle influence you can use to advocace for your brand. The danger is your image. If your community perceives you as a shill, you're done.