Did you know the makers of Scrabulous also had a Chess app?
Yep. It's called Chess Pro and has about 14,000 daily actives users as of today. Now 14,000 DAU isn't bad at all unless you compare it to Scrabulous' 500k+ daily active users.
Here's the thing. Scrabulous ran a link on Scrabulous for about 2-4 weeks (possibly more), saying something like "Try Chess Pro, the new game from the makers of Scrabulous!". It was fairly prominently placed under the Scrabulous playing area. Nothing egregious, Rajat and Jayant are very conservative with their promotional activity, always opting for the understated, classy aesthetic.
Now, how many installs do you think an app cross-promoted from a top ten app would acquire?
You'd probably say "a helluva lot". But of course, you're wrong. As of today, Chess Pro has ~111,000 installs. For perspective, Chess Pro doesn't even rank in the top 1000 for number of installs.
Only 20% of the daily audience of Scrabulous even bothered to install Chess Pro, and arguably less since Chess Pro grew some on its own.
So in this case, I'd suggest that the cross-promotion didn't work.
So how do you effectively cross-promote a game?
By genre. Make sure that players of one game will actually want to play the other game. For instance, there's a significant overlap of users between Scrabulous and Scramble because they are both word games. Yes, it's shocking. People who like to play one word game also like to play another word game. However, they are not necessarily going to want to play Chess. Or Speed Racing.
By demographic. Courtesy of the casual game industry, we know that women like card, puzzle, word, and quiz games. And men like action and strategy games. Obviously there are exceptions, but as a general rule, these stereotypes hold. Use them to your advantage.
And even still, there's no guarantees that the new game will succeed. As Mark Pincus, CEO of Zynga, pointed out at Interplay, (and I paraphrase), no matter how much traffic you sent a game, it won't succeed unless it's good. By good, I think he means viral, since there are a lot of good games with paltry traffic.
The core problem in the social games industry is that making a viral game that's also engaging is really hard. No one is doing it consistently (except Playfish, and even their current model of affixing a light social component to one-player flash games will stale eventually.)
The fact is, if you can't consistently make viral games that are also engaging, then it doesn't matter if you cross-promote from Scrabulous, you're not going to grow your network.