The most important thing, when joining a company, is to enter one with a real engineering culture. Are the most qualified people making important decisions? Is it going to stay that way? Will you be able to make decisions that affect you? (Expecting to make decisions that affect others, in your first 12 months, is asking a big much.) Too many of these hot startups (and almost all of the VC darlings) have shitty MBA culture. Sure, they have free pizza (at 8:00 pm) and engineers can wear jeans, but the major decisions are still made by the wrong sorts of people.
Also, I think the social media bubble is going to embarrass itself just like the last one did, and I think it's going to splotch a lot of resumes. I think it's wiser to make for Real Technology now rather than in 2015 when a large number of people are trying to do the same because Google is no longer acq-hiring IUsedThisToilet apps for $487 million.
Finally, becoming a subordinate employee at a startup is usually a shitty deal. The stock option on 0.01073% of the company isn't real ownership. It's a lottery ticket. And if you don't know what preferences the investors have, you're in absolutely no position to evaluate the equity even if things do go well. (By the way, my friend had an offer at a VC darling, for 60% of the salary he was making but about 0.25% of the company. When he asked if there were preferences against that equity, the offer was rescinded. I don't know if this is normal, but it is telling.)
There are 4 reasons to join a startup. 1. For a promotion. This can be dicey because the startup might hire people above you as it grows, defeating the purpose of that move. If you join a startup as the first programmer and end up answering to an MBA-toting VP before your equity is fully vested, you've been fucked. Startups have good reasons not to make their first programmers CTOs, but if you're the first programmer, you should absolutely insist on investor contact and lock in a VP-level title. 2. You know and trust the people running the operation. I would say that this is the big one. Startups can be amazing opportunities if you trust the people running the show, and if they trust you to make important decisions. 3. Literally no one else is doing that kind of work. Since blue-sky R&D (once funded by large companies and governments) has disappeared or is only available to people with PhDs from top departments, this is not an uncommon case. 4. You're a founder.