The "why sell" question is too easy to answer to even bother with - it was dying on its backside. The early adopters of social media were fond of Bebo, and some creative types put a drama on it, Katemodern, which had a nomination for a Bafta award for innovative use of technology.
That was a lifetime ago in the social web. By the time I wrote my book last year it was more of a footnote - the place you'd find Katemodern by all means but that was meaning less and less to people as the years went past (and another one's gone past now, of course).
No, the question that's more difficult is why anyone would have bought a business which has all but had the last rites read over it. I'm reminded of when Facebook bought Friendfeed - go on, when was the last time you thought of Friendfeed? Nothing seems to have been done with it, although I'd guess it looked like a terrific idea at the time.
My guess is it's going to be the same with Criterion and Bebo. Someone in the buying organisation thinks it's a great idea, maybe they think they've found a niche. Although I wish them well, and it's in nobody's interests that an idea doesn't work out, I won't hold my breath.