After announcing earlier this week that it passed the 100 million user mark, professional networking site LinkedIn sent emails to its early users today thanking them and telling them exactly when they joined.

It’s an ego-flattering gesture, likely designed to provoke responses from eager self-promoters. And that in itself is notable from a site that often comes across as bland and functional — a major handicap in LinkedIn’s push to become a hub for news and conversation. That effort seems to be one of the company’s big goals recently, judging from its IPO filing and new products like LinkedIn Today.

VentureBeat’s Matt Marshall (#14,796) and Dean Takahashi (#73,885) both received emails congratulating them on being among LinkedIn’s first 100,000 users. Judging from the commentary on Twitter, other users are getting messages thanking them for being in the first million.

The email, which is signed by LinkedIn founder Reid Hoffman, says “In any technology adoption lifecycle, there are the innovators, those who help lead the way. That was you.” It looks like many recipients took that message to heart, because I’m seeing a lot of tweets proudly sharing user numbers.

One problem: Most of that commentary isn’t on LinkedIn itself, but rather on Twitter (or at least it seems that way after comparing my news feeds on both sites).

Twitter, of course, is the natural home for this kind of conversation — not only am I seeing tweets with user numbers, but also snarky commentary about those tweets. And Twitter has understood early adopters’ interest in bragging rights for a while now, which is why Twitter clients, such as its iPhone app (but not Twitter.com), prominently feature people’s user numbers in their profiles.

On the other hand, I’m not sure how many Twitter users were really dying to know about their friends’ LinkedIn numbers. So maybe LinkedIn users (the ones who aren’t bragging, anyway) appreciate the relative peace and quiet.