As with blogs, older posts fall off the front page. Now and then, the site administrators will decide that a post should not scroll off into history (for example, an FAQ or an announcement post), so they will configure it to be permanently visible at the top:
What's the verb for doing this? Had you asked me, I would have said that you were pinning the post to the top of the forum. To pin a post is a common enough term, which derives, obviously, from the idea of pinning something to a bulletin board.
A post thusly pinned is also said to be sticky. This is another fairly obvious derivation. The dictionaries I have immediately to hand don't list a definition for sticky that refers to a persistent idea or thought, but that usage is all over the Web, and certainly got a big boost from the book Made to Stick.
So. Using the Magic of Language, you can combine the verbiness of pinning and the adjectiveness of sticky to come up with a new term. Yes you can, as evidenced in a quote from an article on Arstechnica:
I suggest reading our FAQs stickied at the top of the indexing forum to get some ideas of what to do.There are a surprising number of hits for this term. I get around 370K on Bing, 450K on Google. Here's a great example:
Stickied threads are being unstickied!
What's notable, of course, is that the new verb was invented in a roundabout fashion (to stick -> sticky -> to sticky), instead of the plausible usage evolving from simply saying that a post was stuck in a forum. But the new usage has advantages; saying that a post is stuck in a forum is ambiguous, whereas saying that the post is stickied is quite clear.
It's hard to tell whether to sticky is actually used in any form but the participle; it's tricky to search for to sticky as a verb, because of the prevalence of phrases like [lead] to sticky [situations]. If you run across an example of the infinitive or present participle, I'd love to hear about it.
 I must say, the cover of that book, which illustrates a piece of duct tape, was brilliant.